Add a test case to validate code gen for typeof/builtin_types_compatible.

This test case currently generates the following unexpected warnings (when compared with gcc).

[dylan:clang/test/Parser] admin% ../../../../Debug/bin/clang -parse-ast-check builtin_types_compatible.c
Warnings seen but not expected:
  Line 28: expression result unused
  Line 29: expression result unused
  Line 30: expression result unused
  Line 31: expression result unused
  Line 32: expression result unused
  Line 33: expression result unused

llvm-svn: 40789