Loosen -Wempty-body warning

Do not show it when `if` or `else` come from macros.
E.g.,

    #define USED(A) if (A); else
    #define SOME_IF(A) if (A)

    void test() {
      // No warnings are shown in those cases now.
      USED(0);
      SOME_IF(0);
    }

Patch by Ilya Biryukov!

Differential Revision: https://reviews.llvm.org/D40185

llvm-svn: 318556