c++: optimize tsubst_template_decl for function templates

c++: optimize tsubst_template_decl for function templates

r14-2655-g92d1425ca78040 made instantiate_template avoid redundantly
performing a specialization lookup when calling tsubst_decl.  This patch
applies the same optimization to the analagous tsubst_template_decl when
(partially) instantiating a function template.  This allows us to remove
an early exit test from register_specialization since we no longer try
to register the FUNCTION_DECL corresponding to a function template
partial instantiation.

gcc/cp/ChangeLog:

	* pt.cc (register_specialization): Remove now-unnecessary
	early exit for FUNCTION_DECL partial instantiation.
	(tsubst_template_decl): Pass use_spec_table=false to
	tsubst_function_decl.  Set DECL_TI_ARGS of a non-lambda
	FUNCTION_DECL specialization to the full set of arguments.
	Simplify register_specialization call accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/template/nontype12.C: Expect two instead of three
	duplicate diagnostics for A<double>::bar() specialization.