[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361484 91177308-0d34-0410-b5e6-96231b3b80d8

Author: kwk

include/llvm/ADT/DenseMap.h

@@ -63,7 +63,7 @@ struct DenseMapPair : public std::pair<KeyT, ValueT> {
   template <typename AltPairT>
   DenseMapPair(AltPairT &&AltPair,
                typename std::enable_if<std::is_convertible<
-                   AltPairT, std::pair<KeyT, ValueT>>::value>::type * = 0)
+                   AltPairT, std::pair<KeyT, ValueT>>::value>::type * = nullptr)
       : std::pair<KeyT, ValueT>(std::forward<AltPairT>(AltPair)) {}
 
   KeyT &getFirst() { return std::pair<KeyT, ValueT>::first; }

include/llvm/Demangle/MicrosoftDemangleNodes.h

@@ -513,7 +513,7 @@ struct NodeArrayNode : public Node {
 
   void output(OutputStream &OS, OutputFlags Flags, StringView Separator) const;
 
-  Node **Nodes = 0;
+  Node **Nodes = nullptr;
   size_t Count = 0;
 };