c++: Fix up floating point complex handling in build_zero_init_1 [PR98353]

c++: Fix up floating point complex handling in build_zero_init_1 [PR98353]

While the gimplifier patch I've just committed fixed an ICE, in some cases
like on the committed testcase cp folding doesn't happen after
build_zero_init_1 because it is called already during gimplification.

For the scalar types, if we want to use convert, the problem with complex floats
is that it returns a COMPLEX_EXPR with FLOAT_EXPR arguments which have
INTEGER_CST 0 as argument.  As fold isn't recursive, it doesn't do anything
in that case, we need to first fold those FLOAT_EXPRs to REAL_CST 0.0 and
only afterwards the COMPLEX_EXPR can be folded into COMPLEX_CST with 0.0
arguments.

This patch instead just uses build_zero_cst that creates the zero constant for
any scalar types (and more) directly, instead of going through multiple hops.

2020-12-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98353
	* init.c (build_zero_init_1): Use build_zero_cst for SCALAR_TYPE_P
	zero initializers.