[PATCH] MS Compat: interpose vadefs.h to fix definitions of _crt_va_{start, end, arg} (PR21247)

[Reid Kleckner]

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Comment at: lib/Headers/vadefs.h:24
@@ +23,3 @@
+
+/* Only include this if we're compiling for the Windows platform. */
+#ifndef _MSC_VER
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I'd wordsmith this to "Only include this if we're aiming for MSVC compatibility."

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Comment at: lib/Headers/vadefs.h:32
@@ +31,3 @@
+
+#include_next <vadefs.h>
+
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Should we only do this if __has_include(<vadefs.h>) ?

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Comment at: lib/Headers/vadefs.h:34
@@ +33,3 @@
+
+/* Override macros from vadefs.h with definitions that work with Clang. */
+#define _crt_va_start(ap, param) __builtin_va_start(ap, param)
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I think we should do "#ifndef _crt_va_start #undef _crt_va_start" to avoid warnings with -Wsystem-headers.

http://reviews.llvm.org/D5784