Introduce builtin macros to determine whether we're building a

Introduce builtin macros to determine whether we're building a
specific module (__building_module(modulename)) and to get the name of
the current module as an identifier (__MODULE__).

Used to help headers behave differently when they're being included as
part of building a module. Oh, the irony.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164605 91177308-0d34-0410-b5e6-96231b3b80d8