[Sema] Don't mark plain MS enums as fixed

Summary:
This fixes a flaw in our AST: PR27098

MSVC always gives plain enums the underlying type 'int'. Clang does this
as well, but we claim the enum is "fixed", as if the user actually wrote
': int'. It means we end up emitting spurious -Wsign-compare warnings on
code like this:

  enum Vals { E1, E2, E3 };
  bool f(unsigned v1, Vals v2) {
    return v1 == v2;
  }

We think 'v2' can take on negative values because we think 'Vals' is
fixed. This fixes that.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43110

llvm-svn: 324913