Debug info: Implement a cleaner version of r198461. For symmetry with
C and C++ don't emit an extra lexical scope for the compound statement
that is the body of an Objective-C method.

rdar://problem/15010825

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198699 91177308-0d34-0410-b5e6-96231b3b80d8