[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/Expression/IRExecutionUnit.cpp b/lldb/source/Expression/IRExecutionUnit.cpp
index 7c0379c..34a3488 100644
--- a/lldb/source/Expression/IRExecutionUnit.cpp
+++ b/lldb/source/Expression/IRExecutionUnit.cpp
@@ -165,8 +165,8 @@
 
   ArchSpec arch(target->GetArchitecture());
 
-  const char *plugin_name = NULL;
-  const char *flavor_string = NULL;
+  const char *plugin_name = nullptr;
+  const char *flavor_string = nullptr;
   lldb::DisassemblerSP disassembler_sp =
       Disassembler::FindPlugin(arch, flavor_string, plugin_name);
 
@@ -251,7 +251,7 @@
     std::string s;
     llvm::raw_string_ostream oss(s);
 
-    m_module->print(oss, NULL);
+    m_module->print(oss, nullptr);
 
     oss.flush();
 
@@ -839,7 +839,7 @@
     };
 
     if (sc.module_sp) {
-      sc.module_sp->FindFunctions(spec.name, NULL, spec.mask,
+      sc.module_sp->FindFunctions(spec.name, nullptr, spec.mask,
                                   true,  // include_symbols
                                   false, // include_inlines
                                   true,  // append