[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/GDBRemoteCommunicationClient.cpp b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp
index c6b792e..9797184 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp
@@ -386,14 +386,14 @@
         std::vector<std::string> supported_compressions;
         compressions += sizeof("SupportedCompressions=") - 1;
         const char *end_of_compressions = strchr(compressions, ';');
-        if (end_of_compressions == NULL) {
+        if (end_of_compressions == nullptr) {
           end_of_compressions = strchr(compressions, '\0');
         }
         const char *current_compression = compressions;
         while (current_compression < end_of_compressions) {
           const char *next_compression_name = strchr(current_compression, ',');
           const char *end_of_this_word = next_compression_name;
-          if (next_compression_name == NULL ||
+          if (next_compression_name == nullptr ||
               end_of_compressions < next_compression_name) {
             end_of_this_word = end_of_compressions;
           }
@@ -775,7 +775,7 @@
   std::vector<const char *> argv;
   FileSpec exe_file = launch_info.GetExecutableFile();
   std::string exe_path;
-  const char *arg = NULL;
+  const char *arg = nullptr;
   const Args &launch_args = launch_info.GetArguments();
   if (exe_file)
     exe_path = exe_file.GetPath(false);
@@ -786,7 +786,7 @@
   }
   if (!exe_path.empty()) {
     argv.push_back(exe_path.c_str());
-    for (uint32_t i = 1; (arg = launch_args.GetArgumentAtIndex(i)) != NULL;
+    for (uint32_t i = 1; (arg = launch_args.GetArgumentAtIndex(i)) != nullptr;
          ++i) {
       if (arg)
         argv.push_back(arg);
@@ -1094,7 +1094,7 @@
     if (!m_gdb_server_name.empty())
       return m_gdb_server_name.c_str();
   }
-  return NULL;
+  return nullptr;
 }
 
 uint32_t GDBRemoteCommunicationClient::GetGDBServerProgramVersion() {