Improve struct pattern compilation

Improve struct pattern compilation

gcc/rust/ChangeLog:

	* backend/rust-compile-pattern.cc
	(CompilePatternCheckExpr::visit): Fix GENERIC generation in
	light of enum layout changes since this code was written.
	(CompilePatternBindings::handle_struct_pattern_ident_pat):
	Delegate handling of child patterns to another
	CompilePatternBindings::Compile call.
	(CompilePatternBindings::make_struct_access): Make field name
	parameter const qualified.
	* backend/rust-compile-pattern.h
	(CompilePatternBindings::make_struct_access): Likewise.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/struct-pattern-match.rs: New test.

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