The semantic checking that verifies whether an Objective-C method

declared in a subclass has consistent parameter types with a method
having the same selector in a superclass performs a significant number
of lookups into the class hierarchy. In the example in
<rdar://problem/11004361>, we spend 4.7% of -fsyntax-only time in
these lookups.

Optimize away most of the calls to this routine
(Sema::CompareMethodParamsInBaseAndSuper) by first checking whether we
have ever seen *any* method with that selector (using the global
selector table). Since most selectors are unique, we can avoid the
cost of this name lookup in many cases, for a 3.3% speedup.

llvm-svn: 155958