c++: fix string literal member initializer bug [PR90926]

c++: fix string literal member initializer bug [PR90926]

build_aggr_conv did not correctly handle string literal member initializers.
Extended can_convert_array to handle this case. For the additional check of
compatibility of character types, factored out code from digest_init_r into
a new function.

gcc/cp/ChangeLog:

	PR c++/90926
	* call.c (can_convert_array): Extend to handle all valid aggregate
	initializers of an array; including by string literals, not just by
	brace-init-list.
	(build_aggr_conv): Call can_convert_array more often, not just in
	brace-init-list case.
	* typeck2.c (array_string_literal_compatible_p): New function.
	(digest_init_r): call array_string_literal_compatible_p
	* cp-tree.h: (array_string_literal_compatible_p): Declare.

gcc/testsuite/ChangeLog:

	PR c++/90926
	* g++.dg/cpp1y/nsdmi-aggr12.C: New test.