[PATCH] Warn on malformed header guards

[Richard Trieu]

Check that when a header starts with an #ifndef, the next #define has the same macro name.  There's two exceptions, one is that the #define is an actual macro, the other is that the macro is defined later in the file.

// Warn here since x != y
#ifndef x
#define y
#endif

// Don't warn since y is defined to something
#ifndef x
#define y 2
#endif

// Dont warn since x is defined later
#ifndef x
#define y
#define x
#endif

http://llvm-reviews.chandlerc.com/D898

Files:
  test/Lexer/header.cpp
  test/Lexer/Inputs/bad-header-guard.h
  test/Lexer/Inputs/good-header-guard.h
  test/Lexer/Inputs/different-define.h
  test/Lexer/Inputs/no-define.h
  test/Lexer/Inputs/out-of-order-define.h
  test/Lexer/Inputs/multiple.h
  include/clang/Lex/Preprocessor.h
  include/clang/Lex/HeaderSearch.h
  include/clang/Lex/MultipleIncludeOpt.h
  include/clang/Basic/DiagnosticLexKinds.td
  lib/Lex/PPLexerChange.cpp
  lib/Lex/PPDirectives.cpp
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