gccrs: Add error message for AltPattern in let statements

gccrs: Add error message for AltPattern in let statements

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-pattern.cc
	(ASTLoweringPattern::translate): Add is_let_top_level parameter.
	(ASTLoweringPattern::visit): Handle is_let_top_level.
	* hir/rust-ast-lower-pattern.h
	(class ASTLoweringPattern): Add is_let_top_level.
	* hir/rust-ast-lower-stmt.cc
	(ASTLoweringStmt::visit): Set is_let_top_level.

gcc/testsuite/ChangeLog:

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

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