rust: Desugar IfLet* into MatchExpr

rust: Desugar IfLet* into MatchExpr

Replace the "regular" AST->HIR lowering for IfLet* with a desugaring
into a MatchExpr.

Desugar a simple if let:

   if let Some(y) = some_value {
     bar();
   }

into:

   match some_value {
     Some(y) => {bar();},
     _ => ()
   }

Same applies for IfLetExprConseqElse (if let with an else block).

Desugar:

   if let Some(y) = some_value {
     bar();
   } else {
     baz();
   }

into:

   match some_value {
     Some(y) => {bar();},
     _ => {baz();}
   }

Fixes https://github.com/Rust-GCC/gccrs/issues/1177

gcc/rust/ChangeLog:

	* backend/rust-compile-block.h: Adjust after removal of
	HIR::IfLetExpr and HIR::IfLetExprConseqElse.
	* backend/rust-compile-expr.h: Likewise.
	* checks/errors/borrowck/rust-bir-builder-expr-stmt.cc
	(ExprStmtBuilder::visit): Likewise.
	* checks/errors/borrowck/rust-bir-builder-expr-stmt.h: Likewise.
	* checks/errors/borrowck/rust-bir-builder-lazyboolexpr.h:
	Likewise.
	* checks/errors/borrowck/rust-bir-builder-struct.h: Likewise.
	* checks/errors/borrowck/rust-function-collector.h: Likewise.
	* checks/errors/privacy/rust-privacy-reporter.cc
	(PrivacyReporter::visit): Likewise.
	* checks/errors/privacy/rust-privacy-reporter.h: Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::visit):
	Likewise.
	* checks/errors/rust-const-checker.h: Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit):
	Likewise.
	* checks/errors/rust-unsafe-checker.h: Likewise.
	* hir/rust-ast-lower-block.h (ASTLoweringIfLetBlock::translate):
	Change return type.
	* hir/rust-ast-lower.cc (ASTLoweringIfLetBlock::desugar_iflet):
	New.
	(ASTLoweringIfLetBlock::visit(AST::IfLetExpr &)): Adjust and use
	desugar_iflet.
	* hir/rust-ast-lower.h: Add comment.
	* hir/rust-hir-dump.cc (Dump::do_ifletexpr): Remove.
	(Dump::visit(IfLetExpr&)): Remove.
	(Dump::visit(IfLetExprConseqElse&)): Remove.
	* hir/rust-hir-dump.h (Dump::do_ifletexpr): Remove.
	(Dump::visit(IfLetExpr&)): Remove.
	(Dump::visit(IfLetExprConseqElse&)): Remove.
	* hir/tree/rust-hir-expr.h (class IfLetExpr): Remove.
	(class IfLetExprConseqElse): Remove.
	* hir/tree/rust-hir-full-decls.h (class IfLetExpr): Remove.
	(class IfLetExprConseqElse): Remove.
	* hir/tree/rust-hir-visitor.h: Adjust after removal of
	HIR::IfLetExpr and HIR::IfLetExprConseqElse.
	* hir/tree/rust-hir.cc (IfLetExpr::as_string): Remove.
	(IfLetExprConseqElse::as_string): Remove.
	(IfLetExpr::accept_vis): Remove.
	(IfLetExprConseqElse::accept_vis): Remove.
	* hir/tree/rust-hir.h: Adjust after removal of HIR::IfLetExpr and
	HIR::IfLetExprConseqElse.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit):
	Likewise.
	* typecheck/rust-hir-type-check-expr.h: Likewise.
	* checks/errors/rust-hir-pattern-analysis.cc
	(PatternChecker::visit (IfLetExpr &)): Remove.
	(PatternChecker::visit (IfLetExprConseqElse &)): Remove.
	* checks/errors/rust-hir-pattern-analysis.h (visit(IfLetExpr &)): Remove.
	(visit(IfLetExprConseqElse &)): Remove.

gcc/testsuite/ChangeLog:

	* rust/compile/if_let_expr.rs: Adjust.
	* rust/compile/if_let_expr_simple.rs: New test.
	* rust/compile/iflet.rs: New test.
	* rust/execute/torture/iflet.rs: New test.
	* rust/compile/nr2/exclude: Add iflet.rs and if_let_expr_simple.rs

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>