Fixed name lookups for names that contain "::" but aren't actually C++ qualified C++ names.

To do this, I fixed the  CPPLanguageRuntime::StripNamespacesFromVariableName() function to use a regular expression that correctly determines if the name passed to it is a qualfied C++ name like "a::b::c" or "b::c". The old version of this function was treating '__54-[NSUserScriptTask executeWithInterpreter:arguments::]_block_invoke' as a match with a basename of ']_block_invoke'.

Also fixed a case in the by name lookup of functions where we wouldn't look for the full name if we actually tried to call CPPLanguageRuntime::StripNamespacesFromVariableName() and got an empty basename back.

<rdar://problem/18527866>

llvm-svn: 220432