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.

llvm-svn: 57898