[modules] Before marking the module imported macros as ambiguous, check if this is a case where

the system macro uses a not identical definition compared to a macro from the clang headers.

For example (these come from different modules):
   \#define LONG_MAX __LONG_MAX__ (clang's limits.h)
   \#define LONG_MAX 0x7fffffffffffffffL (system's limits.h)
in which case don't mark them ambiguous to avoid the "ambiguous macro expansion" warning.

llvm-svn: 178109