[lldb] Support arm64e C++ vtable pointer signing (#187611)

When targeting arm64e, vtable pointers are signed with a discriminator
that incorporates the object's address
(PointerAuthVTPtrAddressDiscrimination) and class type
(PointerAuthVTPtrTypeDiscrimination).

I had to make a small change to clang, specifically in
getPointerAuthDeclDiscriminator(). Previously, that was computing the
discriminator based on getMangledName(). The latter returns the
AsmLabelAttr, which for functions imported by lldb, is prefixed with
`$__lldb_func`, causing a different discriminator to be generated.

GitOrigin-RevId: 4fe46edf8d9e301c5669250c943377aa1f55fb45