c++: Don't form a templated TARGET_EXPR in finish_compound_literal

c++: Don't form a templated TARGET_EXPR in finish_compound_literal

The atom_cache in normalize_atom relies on the assumption that two
equivalent (templated) trees (in the sense of cp_tree_equal) must use
the same template parameters (according to find_template_parameters).

This assumption unfortunately doesn't always hold for TARGET_EXPRs,
because cp_tree_equal ignores an artificial target of a TARGET_EXPR, but
find_template_parameters walks this target (and its DECL_CONTEXT).

Hence two TARGET_EXPRs built by force_target_expr with the same
initializer and under different settings of current_function_decl will
compare equal according to cp_tree_equal, but find_template_parameters
may return a different set of template parameters for them.  This breaks
the below testcase because during normalization we build two such
TARGET_EXPRs (one under current_function_decl=f and another under =g),
and then share the same ATOMIC_CONSTR for the two corresponding atoms,
leading to a crash during satisfaction of g's associated constraints.

This patch works around this issue by removing the source of these
templated TARGET_EXPRs.  The relevant call to get_target_expr_sfinae was
added in r9-6043, and it seems it's no longer necessary (according to
https://gcc.gnu.org/pipermail/gcc-patches/2019-February/517323.html, the
call was added in order to avoid regressing on initlist109.C at the time).

gcc/cp/ChangeLog:

	* semantics.c (finish_compound_literal): Don't wrap the original
	compound literal in a TARGET_EXPR when inside a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-decltype3.C: New test.