[cfe] [Headers] Fix inttypes.h macros visibility in C++ with C99-compliant libc

[mgorny]

Summary

Define __STDC_FORMAT_MACROS in order to enable all macros defined by inttypes.h in C++ mode, as was required by the C99 standard and was enforced by multiple old glibc versions. This extends the fix from r179419 that covered only macros defined by stdint.h but not those in inttypes.h.

[mgorny]

By the way, if anyone happens to have a 'more final' copy of the standards handy, feel free to provide a more consistent footnote numbers ;-).

[mgorny]

Ping.

[mgorny]

Ping II.

lib/Headers/inttypes.h

 
 #ifndef __CLANG_INTTYPES_H
 #define __CLANG_INTTYPES_H
 
 #if defined(_MSC_VER) && _MSC_VER < 1800
 #error MSVC does not have inttypes.h prior to Visual Studio 2013
 #endif
 
+// C99 7.8.1 Macros for format specifiers
+//
+//  Footnote 191: C++ implementations should define these macros only when
+//  __STDC_FORMAT_MACROS is defined before <inttypes.h> is included.
+//
+// C++11 [c.files]p3:
+//
+//  The macros defined by <cinttypes> are provided unconditionally.
+//  In particular, the symbol __STDC_FORMAT_MACROS (mentioned
+//  in footnote 182 of the C standard) plays no role in C++.
+//
+// C11 removed the problematic footnotes.
+//
+// Work around this inconsistency by always defining those macros in C++ mode,
+// so that a C library implementation which follows the C99 standard can be
+// used in C++.
+
+# if defined(__cplusplus) && !defined(__STDC_FORMAT_MACROS)
+#  define __STDC_FORMAT_MACROS
+#  define __STDC_FORMAT_MACROS_DEFINED_BY_CLANG
+# endif
+
 #include_next <inttypes.h>
 
+# ifdef __STDC_FORMAT_MACROS_DEFINED_BY_CLANG
+#  undef __STDC_FORMAT_MACROS
+#  undef __STDC_FORMAT_MACROS_DEFINED_BY_CLANG
+# endif
+
 #if defined(_MSC_VER) && _MSC_VER < 1900
 /* MSVC headers define int32_t as int, but PRIx32 as "lx" instead of "x".
  * This triggers format warnings, so fix it up here. */
 #undef PRId32
 #undef PRIdLEAST32
 #undef PRIdFAST32
 #undef PRIi32
 #undef PRIiLEAST32