Convert some ObjC msgSends to runtime calls.
It is faster to directly call the ObjC runtime for methods such as retain/release instead of sending a message to those functions.
This patch adds support for converting messages to retain/release/alloc/autorelease to their equivalent runtime calls.
Tests included for the positive case of applying this transformation, negative tests that we ensure we only convert "alloc" to objc_alloc, not "alloc2", and also a driver test to ensure we enable this only for supported runtime versions.
Reviewed by John McCall.
Differential Revision: http://reviews.llvm.org/D14737