gccrs: Parse raw ref operator

gccrs: Parse raw ref operator

The raw ref operator is an unstable feature required to obtain a pointer
to unaligned adresses (mainly unaligned struct fields) without UB.

gcc/rust/ChangeLog:

	* ast/rust-ast-builder.cc (Builder::ref): Adapt constructor to the new
	API.
	* ast/rust-ast-collector.cc (TokenCollector::visit): Emit a raw weak
	keyword when required.
	* ast/rust-ast.cc (BorrowExpr::as_string): Change as_string
	representation to handle raw ref operator.
	* ast/rust-expr.h (class BorrowExpr): Add raw discriminant.
	* expand/rust-macro-builtins-include.cc: Adapt constructor to the new
	API.
	* parse/rust-parse-impl.h: Handle the raw weak keyword.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>