c++: Fix array of char typedef in template (PR90966).

c++: Fix array of char typedef in template (PR90966).

Since Martin Sebor's patch for PR 71625 to change braced array initializers
to STRING_CST in some cases, we need to be ready for STRING_CST with types
that are changed by tsubst.  fold_convert doesn't know how to deal with
STRING_CST, which is reasonable; we really shouldn't expect it to here.  So
let's handle STRING_CST separately.

	PR c++/90966
	* pt.c (tsubst_copy) [STRING_CST]: Don't use fold_convert.