gccrs: hir: Add OffsetOf node

gccrs: hir: Add OffsetOf node

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-expr.h (class OffsetOf): New.
	* hir/tree/rust-hir-expr.cc: Define its methods.
	* hir/tree/rust-hir-expr-abstract.h: Add ExprType::OffsetOf.
	* hir/tree/rust-hir-full-decls.h (class OffsetOf): Declare it.
	* backend/rust-compile-block.h: Add handling for OffsetOf.
	* backend/rust-compile-expr.cc (CompileExpr::visit): Likewise.
	* 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 (RUST_BIR_BUILDER_EXPR_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 (RUST_PRIVACY_REPORTER_H): Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::visit): Likewise.
	* checks/errors/rust-const-checker.h: Likewise.
	* checks/errors/rust-hir-pattern-analysis.cc (PatternChecker::visit): Likewise.
	* checks/errors/rust-hir-pattern-analysis.h: Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit): Likewise.
	* checks/errors/rust-unsafe-checker.h: Likewise.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Likewise.
	* hir/rust-hir-dump.cc (Dump::visit): Likewise.
	* hir/rust-hir-dump.h: Likewise.
	* hir/tree/rust-hir-visitor.cc (DefaultHIRVisitor::walk): Likewise.
	* hir/tree/rust-hir-visitor.h: Likewise.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): Likewise.
	* typecheck/rust-hir-type-check-expr.h (RUST_HIR_TYPE_CHECK_EXPR): Likewise.

gcc/testsuite/ChangeLog:

	* rust/compile/offset_of2.rs: New test.