gccrs: Add mechanism use pattern information to improve type info

When we have an untyped closure we assumed all parameters were inference
variables but we can use the pattern type to try and improve the type info
so if we have a reference pattern it must be a reference to an inference
variables '&_'. This patch adds a new visitor to figure this out for
untyped closure parameters.

Note this test case does fully type resolve into the gimple:

    bool test::main::{{closure}}
        (struct test::main::{{closure}} $closure, struct (& u8) args)
    { ... }

Though the Rustc version does fail type-resolution but we make some
assumptions during comparison expressions here that they resolve to a bool
this will change when we implement the comparison lang items.

Fixes #2142

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-pattern.h: add missing get_mutability
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit):
	use new ClosureParamInfer on untyped parameters
	* typecheck/rust-hir-type-check-pattern.cc (ClosureParamInfer::Resolve): interface
	(ClosureParamInfer::ClosureParamInfer): constructor
	(ClosureParamInfer::visit): visitors for each pattern
	* typecheck/rust-hir-type-check-pattern.h (class ClosureParamInfer): new visitor

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2142.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

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