[lldb] NFC modernize codebase with modernize-use-nullptr

Summary:
NFC = [[ https://llvm.org/docs/Lexicon.html#nfc | Non functional change ]]

This commit is the result of modernizing the LLDB codebase by using
`nullptr` instread of `0` or `NULL`. See
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html
for more information.

This is the command I ran and I to fix and format the code base:

```
run-clang-tidy.py \
	-header-filter='.*' \
	-checks='-*,modernize-use-nullptr' \
	-fix ~/dev/llvm-project/lldb/.* \
	-format \
	-style LLVM \
	-p ~/llvm-builds/debug-ninja-gcc
```

NOTE: There were also changes to `llvm/utils/unittest` but I did not
include them because I felt that maybe this library shall be updated in
isolation somehow.

NOTE: I know this is a rather large commit but it is a nobrainer in most
parts.

Reviewers: martong, espindola, shafik, #lldb, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: arsenm, jvesely, nhaehnle, hiraditya, JDevlieghere, teemperor, rnkovacs, emaste, kubamracek, nemanjai, ki.stfu, javed.absar, arichardson, kbarton, jrtc27, MaskRay, atanasyan, dexonsmith, arphaman, jfb, jsji, jdoerfert, lldb-commits, llvm-commits

Tags: #lldb, #llvm

Differential Revision: https://reviews.llvm.org/D61847

llvm-svn: 361484
diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
index 31af1cd..9c3a02e 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
@@ -98,7 +98,7 @@
   Log *log(ProcessGDBRemoteLog::GetLogIfAllCategoriesSet(GDBR_LOG_PACKETS));
   ConnectionStatus status = eConnectionStatusSuccess;
   char ch = '+';
-  const size_t bytes_written = Write(&ch, 1, status, NULL);
+  const size_t bytes_written = Write(&ch, 1, status, nullptr);
   if (log)
     log->Printf("<%4" PRIu64 "> send packet: %c", (uint64_t)bytes_written, ch);
   m_history.AddPacket(ch, GDBRemoteCommunicationHistory::ePacketTypeSend,
@@ -110,7 +110,7 @@
   Log *log(ProcessGDBRemoteLog::GetLogIfAllCategoriesSet(GDBR_LOG_PACKETS));
   ConnectionStatus status = eConnectionStatusSuccess;
   char ch = '-';
-  const size_t bytes_written = Write(&ch, 1, status, NULL);
+  const size_t bytes_written = Write(&ch, 1, status, nullptr);
   if (log)
     log->Printf("<%4" PRIu64 "> send packet: %c", (uint64_t)bytes_written, ch);
   m_history.AddPacket(ch, GDBRemoteCommunicationHistory::ePacketTypeSend,
@@ -138,7 +138,7 @@
     ConnectionStatus status = eConnectionStatusSuccess;
     const char *packet_data = packet.data();
     const size_t packet_length = packet.size();
-    size_t bytes_written = Write(packet_data, packet_length, status, NULL);
+    size_t bytes_written = Write(packet_data, packet_length, status, nullptr);
     if (log) {
       size_t binary_start_offset = 0;
       if (strncmp(packet_data, "$vFile:pwrite:", strlen("$vFile:pwrite:")) ==
@@ -274,7 +274,7 @@
   Log *log(ProcessGDBRemoteLog::GetLogIfAllCategoriesSet(GDBR_LOG_PACKETS));
 
   // Check for a packet from our cache first without trying any reading...
-  if (CheckForPacket(NULL, 0, packet) != PacketType::Invalid)
+  if (CheckForPacket(nullptr, 0, packet) != PacketType::Invalid)
     return PacketResult::Success;
 
   bool timed_out = false;
@@ -470,7 +470,7 @@
       content_length = hash_mark_idx - content_start;
       std::string bufsize_str(m_bytes.data() + 2, i - 2 - 1);
       errno = 0;
-      decompressed_bufsize = ::strtoul(bufsize_str.c_str(), NULL, 10);
+      decompressed_bufsize = ::strtoul(bufsize_str.c_str(), nullptr, 10);
       if (errno != 0 || decompressed_bufsize == ULONG_MAX) {
         m_bytes.erase(0, size_of_first_packet);
         return false;
@@ -483,7 +483,7 @@
     packet_checksum_cstr[0] = m_bytes[checksum_idx];
     packet_checksum_cstr[1] = m_bytes[checksum_idx + 1];
     packet_checksum_cstr[2] = '\0';
-    long packet_checksum = strtol(packet_checksum_cstr, NULL, 16);
+    long packet_checksum = strtol(packet_checksum_cstr, nullptr, 16);
 
     long actual_checksum = CalculcateChecksum(
         llvm::StringRef(m_bytes).substr(1, hash_mark_idx - 1));
@@ -848,7 +848,7 @@
             ::isxdigit(m_bytes[checksum_idx + 1])) {
           if (GetSendAcks()) {
             const char *packet_checksum_cstr = &m_bytes[checksum_idx];
-            char packet_checksum = strtol(packet_checksum_cstr, NULL, 16);
+            char packet_checksum = strtol(packet_checksum_cstr, nullptr, 16);
             char actual_checksum = CalculcateChecksum(
                 llvm::StringRef(m_bytes).slice(content_start, content_end));
             success = packet_checksum == actual_checksum;
@@ -923,9 +923,9 @@
     // Do the listen on another thread so we can continue on...
     if (connection->Connect(comm->m_listen_url.c_str(), &error) !=
         eConnectionStatusSuccess)
-      comm->SetConnection(NULL);
+      comm->SetConnection(nullptr);
   }
-  return NULL;
+  return nullptr;
 }
 
 Status GDBRemoteCommunication::StartDebugserverProcess(