gccrs: implement the TuplePattern and use it for function patterns

In order to handle the tuple pattern of: fn test ((x _) : (i32, i32)) -> i32 { x }
we need to recognize that ABI wise this function still takes a tuple as the parameter
to this function its just how we can address the "pattern" of the tuple changes.

So reall if this was C it would look like:

  void test (struct tuple_type __prameter)
  {
    return __parameter.0
  }

The code here reuses our existing pattern code so that we generate these implicit
bindings of the paramter with a field access so any time x is referenced it's really
just emplacing __parameter.0 for the field access into the struct which is a tuple.

Fixes Rust-GCC#2847

gcc/rust/ChangeLog:

	* backend/rust-compile-fnparam.cc (CompileFnParam::visit): compile tuple patterns
	(CompileSelfParam::compile): update return type
	(CompileFnParam::create_tmp_param_var): return Bvariable not tree to stop ICE
	* backend/rust-compile-fnparam.h: update prototype
	* backend/rust-compile-pattern.cc (CompilePatternBindings::visit): implement TuplePattern
	* backend/rust-compile-pattern.h: update prototype

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2847.rs: New test.

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

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