c++: Remove LOOKUP_CONSTINIT.
Since we now have DECL_DECLARED_CONSTINIT_P, we no longer need
LOOKUP_CONSTINIT.
gcc/cp/ChangeLog:
* cp-tree.h (LOOKUP_CONSTINIT): Remove.
(LOOKUP_REWRITTEN): Adjust.
* decl.c (duplicate_decls): Set DECL_DECLARED_CONSTINIT_P.
(check_initializer): Use DECL_DECLARED_CONSTINIT_P instead of
LOOKUP_CONSTINIT.
(cp_finish_decl): Don't set DECL_DECLARED_CONSTINIT_P. Use
DECL_DECLARED_CONSTINIT_P instead of LOOKUP_CONSTINIT.
(grokdeclarator): Set DECL_DECLARED_CONSTINIT_P.
* decl2.c (grokfield): Don't handle LOOKUP_CONSTINIT.
* parser.c (cp_parser_decomposition_declaration): Remove
LOOKUP_CONSTINIT handling.
(cp_parser_init_declarator): Likewise.
* pt.c (tsubst_expr): Likewise.
(instantiate_decl): Likewise.
* typeck2.c (store_init_value): Use DECL_DECLARED_CONSTINIT_P instead
of LOOKUP_CONSTINIT.