Fix <rdar://problem/6261178> clang-on-xcode: [sema] multiple method warning is over...

Fix <rdar://problem/6261178> clang-on-xcode: [sema] multiple method warning is over enthusiastic.
Fix <rdar://problem/6265257> warnings for ambiguous message send swamp other warnings.

Reworked Sema::MatchTwoMethodDeclarations() to optionally match based on method size and alignment (the default in GCC). Changed Sema::LookupInstanceMethodInGlobalPool() to use this feature.

Added -Wno-struct-selector-match to driver, however didn't hook it up yet. Added a FIXME that says this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57898 91177308-0d34-0410-b5e6-96231b3b80d8