Convert some ObjC msgSends to runtime calls.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263607 91177308-0d34-0410-b5e6-96231b3b80d8