[clang] Don't use the AST to display backend diagnostics

We keep a map from function name to source location so we don't have to
do it via looking up a source location from the AST. However, since
function names can be long, we actually use a hash of the function name
as the key.

Additionally, we can't rely on Clang's printing of function names via
the AST, so we just demangle the name instead.

This is necessary to implement
https://lists.llvm.org/pipermail/cfe-dev/2021-September/068930.html.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D110665