Add C parser support for "restrict" and "_Atomic"

A user noticed that "watch -location" would fail with a "restrict"
pointer.  The issue here is that if the DWARF mentions "restrict", gdb
will put this into the type name -- but then the C parser will not be
able to parse this type.

This patch adds support for "restrict" and "_Atomic" to the C parser.
C++ doesn't have "restrict", but does have some GCC extensions.  The
type printer is changed to handle this difference as well, so that
watch expressions will work properly.

gdb/ChangeLog
2020-03-14  Tom Tromey  <tom@tromey.com>

	* c-typeprint.c (cp_type_print_method_args): Print "__restrict__"
	for C++.
	(c_type_print_modifier): Likewise.  Add "language" parameter.
	(c_type_print_varspec_prefix, c_type_print_base_struct_union)
	(c_type_print_base_1): Update.
	* type-stack.h (enum type_pieces) <tp_atomic, tp_restrict>: New
	constants.
	* type-stack.c (type_stack::insert): Handle tp_atomic and
	tp_restrict.
	(type_stack::follow_type_instance_flags): Likewise.
	(type_stack::follow_types): Likewise.  Merge type-following code.
	* c-exp.y (RESTRICT, ATOMIC): New tokens.
	(space_identifier, cv_with_space_id)
	(const_or_volatile_or_space_identifier_noopt)
	(const_or_volatile_or_space_identifier): Remove.
	(single_qualifier, qualifier_seq_noopt, qualifier_seq): New
	rules.
	(ptr_operator, typebase): Update.
	(enum token_flag) <FLAG_C>: New constant.
	(ident_tokens): Add "restrict", "__restrict__", "__restrict", and
	"_Atomic".
	(lex_one_token): Handle FLAG_C.

gdb/testsuite/ChangeLog
2020-03-14  Tom Tromey  <tom@tromey.com>

	* gdb.base/cvexpr.exp: Add test for _Atomic and restrict.

gdb/type-stack.h

@@ -40,6 +40,8 @@ enum type_pieces
     tp_const, 
     tp_volatile, 
     tp_space_identifier,
+    tp_atomic,
+    tp_restrict,
     tp_type_stack,
     tp_kind
   };