Improve crash recovery cleanup to recovery CompilerInstances during crash...

Improve crash recovery cleanup to recovery CompilerInstances during crash recovery.  This was a huge resource "root" during crashes.

This change requires making a bunch of fundamental Clang structures (optionally) reference counted to allow correct
ownership semantics of these objects (e.g., ASTContext) to play out between an active ASTUnit and CompilerInstance
object.

llvm-svn: 128011