PR17567: Improve diagnostic for a mistyped constructor name. If we see something

that looks like a function declaration, except that it's missing a return type,
try typo-correcting it to the relevant constructor name.

In passing, fix a bug where the missing-type-specifier recovery codepath would
drop a preceding scope specifier on the floor, leading to follow-on diagnostics
and incorrect recovery for the auto-in-c++98 hack.

llvm-svn: 192644