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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@101088 91177308-0d34-0410-b5e6-96231b3b80d8