[AST] Fix -ast-print for _Bool when have diagnostics

For example, given:

  #define bool _Bool
  _Bool i;
  void fn() { 1; }

-ast-print produced:

  tmp.c:3:13: warning: expression result unused
  void fn() { 1; }
              ^
  bool i;
  void fn() {
      1;
  }

That fails to compile because bool is undefined.

Details:

Diagnostics print _Bool as bool when the latter is defined as the
former.  However, diagnostics were altering the printing policy for
-ast-print as well.  The printed source was then invalid because the
preprocessor eats the bool definition.

Problematic diagnostics included suppressed warnings (e.g., add
-Wno-unused-value to the above example), including those that are
suppressed by default.

This patch fixes this bug and cleans up some related comments.

Reviewed by: aaron.ballman, rsmith

Differential Revision: https://reviews.llvm.org/D45093

llvm-svn: 332275