Avoid names like __in that conflict with SAL in builtin headers

Microsoft's Source Annotation Language (SAL) defines a bunch of keywords
for annotating the inputs and outputs of functions.  Empty definitions
for the keywords are provided by <stdlib.h> -> <crtdefs.h> -> <sal.h>.
This makes it basically impossible to include MSVC's stdlib.h and
Clang's *mmintrin.h headers at the same time if they have variables
named __in.  As a workaround, I've renamed those variables.

This fixes the Modules/compiler_builtins.m test which was XFAILed,
presumably due to this conflict.

llvm-svn: 179860