Change CXTranslationUnit to not directly cast to an ASTUnit*,

but to wrap both an ASTUnit and a "string pool"
that will be used for fast USR generation.

This requires a bunch of mechanical changes, as
there was a ton of code that assumed that CXTranslationUnit
and ASTUnit* were the same.

Along with this change, introduce CXStringBuf,
which provides an llvm::SmallVector<char> backing
for repeatedly generating CXStrings without a huge
amount of malloc() traffic.  This requires making
some changes to the representation of CXString
by renaming a few fields (but keeping the size
of the object the same).

llvm-svn: 119337