gccrs: Unify AST::IfLetExprConseqIf{,Let} into AST::IfLetExprConseqElse

gccrs: Unify AST::IfLetExprConseqIf{,Let} into AST::IfLetExprConseqElse

This simplifies the AST's representation of if-let-statements
to match the HIR.

gcc/rust/ChangeLog:

	* ast/rust-expr.h
	(class IfLetExprConseqElse): Make else_block ExprWithBlock.
	(class IfLetExprConseqIf): Remove.
	(class IfLetExprConseqIfLet): Remove.
	* ast/rust-ast-full-decls.h
	(class IfLetExprConseqIf): Remove.
	(class IfLetExprConseqIfLet): Remove.
	* ast/rust-ast.cc
	(IfLetExprConseqElse::as_string): Adjust output.
	(IfLetExprConseqIf::as_string): Remove.
	(IfLetExprConseqIfLet::as_string): Remove.
	(IfLetExprConseqIf::accept_vis): Remove.
	(IfLetExprConseqIfLet::accept_vis): Remove.
	* ast/rust-ast-visitor.h
	(ASTVisitor::visit): Remove IfLetExprConseqIf{,Let} visitors.
	* ast/rust-ast-dump.cc
	(Dump::visit): Likewise.
	* ast/rust-ast-dump.h:
	(Dump::visit): Likewise.
	* ast/rust-ast-tokenstream.cc
	(TokenStream::visit): Likewise.
	* ast/rust-ast-tokenstream.h
	(TokenStream::visit): Likewise.
	* util/rust-attributes.cc
	(AttributeChecker::visit): Likewise.
	* util/rust-attributes.h:
	(AttributeChecker::visit): Likewise.
	* resolve/rust-early-name-resolver.cc
	(EarlyNameResolver::visit): Likewise.
	* resolve/rust-early-name-resolver.h
	(EarlyNameResolver::visit): Likewise.
	* resolve/rust-ast-resolve-base.h
	(ResolverBase::visit): Likewise.
	* resolve/rust-ast-resolve-base.cc
	(ResolverBase::visit): Likewise.
	* checks/errors/rust-feature-gate.h
	(FeatureGate::visit): Likewise.
	* expand/rust-cfg-strip.cc
	(CfgStrip::visit): Likewise.
	* expand/rust-cfg-strip.h:
	(CfgStrip::visit): Likewise.
	* expand/rust-expand-visitor.cc
	(ExpandVisitor::visit): Likewise.
	* expand/rust-expand-visitor.h
	(ExpandVisitor::visit): Likewise.
	* hir/rust-ast-lower-base.cc
	(ASTLoweringBase::visit): Likewise.
	* hir/rust-ast-lower-base.h:
	(ASTLoweringBase::visit): Likewise.
	* parse/rust-parse-impl.h
	(Parser::parse_if_let_expr): Replace IfLetExprConseqIf{,Let} with IfLetExprConseqElse.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>