Consolidate and improve the handling of built-in feature-like macros

Summary:
The parsing logic has been separated out from the macro implementation logic, leading to a number of improvements:

* Gracefully handle unexpected/invalid tokens, too few, too many and nested parameters
* Provide consistent behaviour between all built-in feature-like macros
* Simplify the implementation of macro logic
* Fix __is_identifier to correctly return '0' for non-identifiers

Reviewers: doug.gregor, rsmith

Subscribers: rsmith, cfe-commits

Differential Revision: http://reviews.llvm.org/D17149

llvm-svn: 265381

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -401,6 +401,7 @@ def err_pp_expected_rparen : Error<"expected ')' in preprocessor expression">;
 def err_pp_expected_eol : Error<
   "expected end of line in preprocessor expression">;
 def err_pp_expected_after : Error<"missing %1 after %0">;
+def err_pp_nested_paren : Error<"nested parentheses not permitted in %0">;
 def err_pp_colon_without_question : Error<"':' without preceding '?'">;
 def err_pp_division_by_zero : Error<
   "division by zero in preprocessor expression">;