c++: Fix up tsubst of BIT_CAST_EXPR [PR98329]

c++: Fix up tsubst of BIT_CAST_EXPR [PR98329]

As the testcase shows, calling cp_build_bit_cast in tsubst_copy doesn't seem
to be a good idea, because tsubst_copy might not really make the operand
non-dependent, but as processing_template_decl can be 0,
type_dependent_expression_p will return false and then cp_build_bit_cast
assumes the type is non-NULL and non-dependent.
So, this patch just follows what is done e.g. for NOP_EXPR etc. and just
builds some tree in tsubst_copy, and only calls the semantics.c function
from tsubst_copy_and_build.

2021-01-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98329
	* pt.c (tsubst_copy) <case BIT_CAST_EXPR>: Don't call
	cp_build_bit_cast here, instead just build_min a BIT_CAST_EXPR and set
	its location.
	(tsubst_copy_and_build): Handle BIT_CAST_EXPR.

	* g++.dg/cpp2a/bit-cast10.C: New test.