gccrs: Add AST to HIR lowering pass

gccrs: Add AST to HIR lowering pass

This performs the lowering of the AST to HIR. The interesting piece here is
that we desugar much of the AST as mentioned in the previous pass, but
crucially, we also strip out all code that is "marked-for-strip" which failed
cfg-expansion from the expansion pass. After this, the HIR includes all code
required to compile for this crate.

	gcc/rust/
	* hir/rust-ast-lower-base.cc: New.
	* hir/rust-ast-lower-base.h: New.
	* hir/rust-ast-lower-block.h: New.
	* hir/rust-ast-lower-enumitem.h: New.
	* hir/rust-ast-lower-expr.h: New.
	* hir/rust-ast-lower-extern.h: New.
	* hir/rust-ast-lower-implitem.h: New.
	* hir/rust-ast-lower-item.cc: New.
	* hir/rust-ast-lower-item.h: New.
	* hir/rust-ast-lower-pattern.cc: New.
	* hir/rust-ast-lower-pattern.h: New.
	* hir/rust-ast-lower-stmt.h: New.
	* hir/rust-ast-lower-struct-field-expr.h: New.
	* hir/rust-ast-lower-type.h: New.
	* hir/rust-ast-lower.cc: New.
	* hir/rust-ast-lower.h: New.
	* hir/rust-hir-dump.cc: New.
	* hir/rust-hir-dump.h: New.