c++: non-dependent call to consteval operator [PR105912]

Here we're crashing when substituting a non-dependent call to a
consteval operator, whose CALL_EXPR_OPERATOR_SYNTAX flag we try to
propagate to the result, but the result isn't a CALL_EXPR since the
selected function is consteval.  This patch fixes this by checking the
result of extract_call_expr accordingly.  (Note that we can't check
DECL_IMMEDIATE_FUNCTION_P here because we don't know which function was
selected by overload resolution from here.)

	PR c++/105912

gcc/cp/ChangeLog:

	* call.cc (extract_call_expr): Return a NULL_TREE on failure
	instead of asserting.
	* pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Guard against
	NULL_TREE extract_call_expr result.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval31.C: New test.

(cherry picked from commit f70c18524221dcefa6cd26cee7b55503181bd912)

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