Fix TestStopHookMultipleThreads and TestNamespace after r241751

The mentioned commit introduced a subtle change in behavior when printing variable names. This
occured when we have a variable, for which we only know the demangled name, because the compiler
has failed to provide one (this typically happens for variables in anonymous namespaces). A
Mangled class which contains only a demangled name considers itself to be invalid (this could
possibly be a bug), but it's GetName() method still returns a valid demangled name. The previous
commit introduced the check for the validity of the class, and if it failed, it would fall back
to printing the bare name (without the namespace prefixes, as the tests were expecting). I revert
this part of the commit and check the validity of the string returned by GetName() instead.

llvm-svn: 241795