Support decimal floating-point constants in C2x.

Support decimal floating-point constants in C2x.

ISO C2x adds decimal floating point as an optional standard feature.
This patch accordingly makes GCC accept DFP constants (DF, DD, DL, df,
dd, dl suffixes) in strict C2X mode, with a pedwarn-if-pedantic for
older standards and a warning with -Wc11-c2x-compat even in C2x mode
(which in turn requires -Wc11-c2x-compat to be newly passed through to
libcpp).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/c-family:
	* c.opt (Wc11-c2x-compat): Add CPP(cpp_warn_c11_c2x_compat)
	CppReason(CPP_W_C11_C2X_COMPAT).

gcc/testsuite:
	* gcc.dg/dfp/c11-constants-1.c, gcc.dg/dfp/c11-constants-2.c,
	gcc.dg/dfp/c2x-constants-1.c, gcc.dg/dfp/c2x-constants-2.c: New
	tests.
	* gcc.dg/dfp/constants-pedantic.c: Use -std=gnu17 explicitly.
	Update expected diagnostics.

libcpp:
	* include/cpplib.h (struct cpp_options): Add dfp_constants and
	cpp_warn_c11_c2x_compat.
	(enum cpp_warning_reason): Add CPP_W_C11_C2X_COMPAT.
	* init.c (struct lang_flags): Add dfp_constants.
	(lang_defaults): Set dfp_constants to 1 for GNUC2X and STDC2X and
	0 for other languages.
	(cpp_set_lang): Set dfp_constants from language.
	(cpp_create_reader): Set cpp_warn_c11_c2x_compat to -1.
	* expr.c (interpret_float_suffix): Mention DFP constants as C2X in
	comment.
	(cpp_classify_number): Do not diagnose DFP constants for languages
	setting dfp_constants, unless cpp_warn_c11_c2x_compat.

From-SVN: r276908