Fix rdar://6814950 - stdint.h isn't "-pedantic -std=c89" clean,

by marking the predefines buffer as a system header.  The problem 
with stdint is that it was getting problems like this:

/Volumes/Projects/cvs/llvm/Debug/lib/clang/1.0/include/stdint.h:43:9: warning: 'long long' is an extension when C99 mode is not enabled
typedef __INT64_TYPE__ int64_t;
        ^
<built-in>:73:29: note: instantiated from:
#define __INT64_TYPE__ long long
                            ^

We correctly silence warnings in system headers, but only if the 
spelling location of the token came from the system header.  This is
designed so that if you use a system macro in your code that you don't
get punished for its definition.  This is all cool except that the 
predefines buffer wasn't considered a system header.

llvm-svn: 69770