[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/Utility/StopInfoMachException.cpp b/lldb/source/Plugins/Process/Utility/StopInfoMachException.cpp
index 64b88e6..588015a 100644
--- a/lldb/source/Plugins/Process/Utility/StopInfoMachException.cpp
+++ b/lldb/source/Plugins/Process/Utility/StopInfoMachException.cpp
@@ -37,11 +37,11 @@
         target ? target->GetArchitecture().GetMachine()
                : llvm::Triple::UnknownArch;
 
-    const char *exc_desc = NULL;
+    const char *exc_desc = nullptr;
     const char *code_label = "code";
-    const char *code_desc = NULL;
+    const char *code_desc = nullptr;
     const char *subcode_label = "subcode";
-    const char *subcode_desc = NULL;
+    const char *subcode_desc = nullptr;
 
 #if defined(__APPLE__)
     char code_desc_buf[32];
@@ -593,7 +593,7 @@
           // the thread ID so we must always report the breakpoint regardless
           // of the thread.
           if (bp_site_sp->ValidForThisThread(&thread) ||
-              thread.GetProcess()->GetOperatingSystem() != NULL)
+              thread.GetProcess()->GetOperatingSystem() != nullptr)
             return StopInfo::CreateStopReasonWithBreakpointSiteID(
                 thread, bp_site_sp->GetID());
           else if (is_trace_if_actual_breakpoint_missing)