glsl: Store the precision for a function return type

The precision for a function return type is now stored in
ir_function_signature. This will later be useful to implement mediump
to float16 lowering. In the meantime it is also useful to catch errors
where a function is redeclared with a different precision.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/ir.h

@@ -1222,7 +1222,7 @@ public:
    /**
     * Function return type.
     *
-    * \note This discards the optional precision qualifier.
+    * \note The precision qualifier is stored separately in return_precision.
     */
    const struct glsl_type *return_type;
 
@@ -1237,6 +1237,13 @@ public:
    /** Whether or not this function has a body (which may be empty). */
    unsigned is_defined:1;
 
+   /*
+    * Precision qualifier for the return type.
+    *
+    * See the comment for ir_variable_data::precision for more details.
+    */
+   unsigned return_precision:2;
+
    /** Whether or not this function signature is a built-in. */
    bool is_builtin() const;