d: Fix ICE in finish_thunk (PR97644)

Because this what the upstream reference compiler did, thunks for the D
front-end were associated with the class definition, so were forced
code-gen even if the target function was extern.  This has now been
changed so there are now only generated if there is a function
definition, fixing the ICE that occurred in PR 97644, which was caused
by calling expand_thunk() early.

gcc/d/ChangeLog:

	PR d/97644
	* dmd/MERGE: Merge upstream dmd 95044d8e4.
	* d-target.cc (TargetCPP::thunkMangle): New function.
	* decl.cc (finish_thunk): Don't force expand thunks for external
	functions.
	(make_thunk): Emit thunks only if the function has a definition.
	Generate correct mangling for thunks to C++ classes.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr92216.d: Update scan-assember.