[MSVC Compat] Try to treat an implicit, fixed enum as an unfixed enum

consider the following:
enum E *p;
enum E { e };

The above snippet is not ANSI C because 'enum E' has not bee defined
when we are processing the declaration of 'p'; however, it is a popular
extension to make the above work.  This would fail using the Microsoft
enum semantics because the definition of 'E' would implicitly have a
fixed underlying type of 'int' which would trigger diagnostic messages
about a mismatch between the declaration and the definition.

Instead, treat fixed underlying types as not fixed for the purposes of
the diagnostic.

llvm-svn: 249674