Hack Sema::LookupDeclInScope() to avoid calling Sema::LookupName() when parsing C/ObjC.

This results in a 1.7% improvement for "Cocoa.h". If we can figure out how to return a "Decl *", rather than a Sema::LookupResult(), we will likely bump the speedup from 1.7%->2.5%. I verified this, however couldn't get it to work without breaking a fair number of C++ test cases. Will discuss with Doug offline.

llvm-svn: 63320