Improve source-location information for C++ conversion functions, by

copying the type location information from the conversion-type-id into
the type location information for the function type. Do something
similar for constructors and destructors, by giving their "void"
return type source-location information.

In all of these cases, we previously left this type-source information
uninitialized, which led to various unfortunate crashes.

We still aren't tracking good source-location information for the
actual names. That's PR6357.

John, please check my sanity on this.

llvm-svn: 101088