c++: Fix ICE on complex constant with -frounding-math [PR103114]

c++: Fix ICE on complex constant with -frounding-math [PR103114]

The FE uses build_complex which assumes that fold_convert will fold
value to a constant.  With -frounding-math that isn't guaranteed though.
So, the patch instead fold_build2s COMPLEX_EXPR, which will result
in build_complex if both arguments are constants, and otherwise
will build COMPLEX_EXPR.
build_zero_cst is an optimization for fold_convert (type, integer_zero_node).

2021-11-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103114
	* parser.c (cp_parser_userdef_numeric_literal): Use fold_build2
	with COMPLEX_EXPR arg instead of build_complex, use build_zero_cst
	instead of fold_convert from integer_zero_node.

	* g++.dg/ext/complex10.C: New test.