Editor code: tidy up BOOLs,NULLs and overrides pt4.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 4 of a split #2847

Signed-off-by: Nemerle <nemerle5+git@gmail.com>

Code/Editor/Util/IndexedFiles.cpp

@@ -14,7 +14,7 @@
 #include "IndexedFiles.h"
 
 volatile TIntAtomic CIndexedFiles::s_bIndexingDone;
-CIndexedFiles* CIndexedFiles::s_pIndexedFiles = NULL;
+CIndexedFiles* CIndexedFiles::s_pIndexedFiles = nullptr;
 
 bool CIndexedFiles::m_startedFileIndexing = false;