Fold __builtin_memchr function

Fold __builtin_memchr function

	* builtins.h(target_char_cst_p): Declare the function.
	* builtins.c (fold_builtin_memchr): Remove.
	(target_char_cst_p): Move the function from gimple-fold.c.
	(fold_builtin_3): Do not call the function.
	* gimple-fold.c (gimple_fold_builtin_memchr): New function.
	(gimple_fold_builtin): Call the function.
	* fold-const-call.c (fold_const_call_1): Handle CFN_BUILT_IN_MEMCHR.

From-SVN: r241160