c++: Check DECL_TEMPLATE_PARM_P in duplicate_decls [PR97511]

c++: Check DECL_TEMPLATE_PARM_P in duplicate_decls [PR97511]

This makes duplicate_decls differentiate a TYPE_DECL for an alias
template from a TYPE_DECL for one of its template parameters.  The
recently added assert in template_parm_to_arg revealed this latent issue
because merging of the two TYPE_DECLs cleared the DECL_TEMPLATE_PARM_P
flag.

With this patch, we now also correctly diagnose the name shadowing in
the below testcase (as required by [temp.local]/6).

gcc/cp/ChangeLog:

	PR c++/97511
	* decl.c (duplicate_decls): Return NULL_TREE if
	DECL_TEMPLATE_PARM_P differ.

gcc/testsuite/ChangeLog:

	PR c++/97511
	* g++.dg/template/shadow3.C: New test.