c: Handle C_TYPE_INCOMPLETE_VARS even for ENUMERAL_TYPEs [PR94172]

c: Handle C_TYPE_INCOMPLETE_VARS even for ENUMERAL_TYPEs [PR94172]

The following testcases ICE, because they contain extern variable
declarations with incomplete enum types that is later completed and after
that those variables are accessed.  The ICEs are because the vars then may have
incorrect DECL_MODE etc., e.g. in the first case the var has SImode
DECL_MODE (the guessed mode for the enum), but the enum then actually has
DImode because its enumerators don't fit into unsigned int.

The following patch fixes it by using C_TYPE_INCOMPLETE_VARS not just on
incomplete struct/union types, but also incomplete enum types.
TYPE_VFIELD can't be used as it is TYPE_MIN_VALUE on ENUMERAL_TYPE,
thankfully TYPE_LANG_SLOT_1 has been used in the C FE only on
FUNCTION_TYPEs.

2020-03-17  Jakub Jelinek  <jakub@redhat.com>

	PR c/94172
	* c-tree.h (C_TYPE_INCOMPLETE_VARS): Define to TYPE_LANG_SLOT_1
	instead of TYPE_VFIELD, and support it on {RECORD,UNION,ENUMERAL}_TYPE.
	(TYPE_ACTUAL_ARG_TYPES): Check that it is only used on FUNCTION_TYPEs.
	* c-decl.c (pushdecl): Push C_TYPE_INCOMPLETE_VARS also to
	ENUMERAL_TYPEs.
	(finish_incomplete_vars): New function, moved from finish_struct.  Use
	relayout_decl instead of layout_decl.
	(finish_struct): Remove obsolete comment about C_TYPE_INCOMPLETE_VARS
	being TYPE_VFIELD.  Use finish_incomplete_vars.
	(finish_enum): Clear C_TYPE_INCOMPLETE_VARS.  Call
	finish_incomplete_vars.
	* c-typeck.c (c_build_qualified_type): Clear C_TYPE_INCOMPLETE_VARS
	also on ENUMERAL_TYPEs.

	* gcc.dg/pr94172-1.c: New test.
	* gcc.dg/pr94172-2.c: New test.