ir_function_signature: Add method to get the function owning a signature

There is no setter function, the getter returns a constant pointer,
and ir_function_signature::_function is private for a reason.  The
only way to make a connection between a function and function
signature is via ir_function::add_signature.  This helps ensure that
certain invariants (i.e., a function signature is in the list of
signatures for its _function) are met.

src/glsl/ir.cpp

@@ -771,7 +771,7 @@ ir_variable::component_slots() const
 
 
 ir_function_signature::ir_function_signature(const glsl_type *return_type)
-   : return_type(return_type), is_defined(false)
+   : return_type(return_type), is_defined(false), _function(NULL)
 {
    /* empty */
 }