Follow-up to r217302 and r227555: Don't crash on inline ~A::A() if A is an int.

Even with r227555, this still crashed:

  struct S {
    int A;
    ~A::A() {}
  };

That's because ParseOptionalCXXScopeSpecifier()'s call to
ActOnCXXNestedNameSpecifier() doesn't mark the scope spec as invalid if sema
thought it's a good idea to fixit-correct "::" to ":".  For the diagnostic
improvement done in r217302, we never want :: to be interpreted as :, so fix
this by setting ColonSacred to false temporarily.

Found by SLi's bot.

llvm-svn: 227581