[preprocessor] Don't warn about "disabled expansion of recursive macro"
for "#define X X".

This is a pattern that, for example, stdbool.h uses.
rdar://12435773

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173952 91177308-0d34-0410-b5e6-96231b3b80d8