Fixed a problem where maintaining the ObjCInterfaceMap

for each ObjCInterfaceDecl was imposing performance
penalties for Objective-C apps.  Instead, we now use
the normal function query mechanisms, which use the
relevant accelerator tables.

This fix also includes some modifications to the
SymbolFile which allow us to find Objective-C methods
and report their Clang Decls correctly.

llvm-svn: 148457