Pull in r353907 from upstream llvm trunk (by Reid Kleckner):

Pull in r353907 from upstream llvm trunk (by Reid Kleckner):

  [MC] Make symbol version errors non-fatal

  We stil don't have a source location, which is pretty lame, but at
  least we won't tell the user to file a clang bug report anymore.

  Fixes PR40712

This will make errors for symbols with @@ versions that are not defined
non-fatal.  For example:

  void f(void)
  {
    __asm__(".symver foo,bar@@baz");
  }

will now result in:

  error: versioned symbol bar@@baz must be defined

instead of clang crashing with a diagnostic report.

PR:		234671
Upstream PR:	https://bugs.llvm.org/show_bug.cgi?id=40712
MFC after:	3 days