Remove _clang_initCXLookupHint() and _clang_getCursorWithHint(). Related to <rdar://problem/7310688>.

Localize the optimization to ResolveLocationInAST(). The last valid AST location is now stored with ASTUnit. There still isn't optimal, however it's an improvement (with a much cleaner API). Having the client manage an "hint" is error prone and complex.

I wanted to land the major changes before finishing up the optimizations. 

llvm-svn: 85425