[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

lldb/source/API/SBValueList.cpp

@@ -92,7 +92,7 @@ bool SBValueList::IsValid() const {
 SBValueList::operator bool() const {
   LLDB_RECORD_METHOD_CONST_NO_ARGS(bool, SBValueList, operator bool);
 
-  return (m_opaque_up != NULL);
+  return (m_opaque_up != nullptr);
 }
 
 void SBValueList::Clear() {
@@ -172,7 +172,7 @@ uint32_t SBValueList::GetSize() const {
 }
 
 void SBValueList::CreateIfNeeded() {
-  if (m_opaque_up == NULL)
+  if (m_opaque_up == nullptr)
     m_opaque_up.reset(new ValueListImpl());
 }