GLSL/SPV: Propagaet precision qualifier from function to return value.

When a return value's type has no precision qualification (e.g., the return
expression is formed from a constructor), and the formal function return type
has a precision qualification, back propagate that from the return type to the
type of the return value's expression.

glslang/Include/intermediate.h

@@ -1231,6 +1231,7 @@ public:
     TOperator getFlowOp() const { return flowOp; }
     TIntermTyped* getExpression() const { return expression; }
     void setExpression(TIntermTyped* pExpression) { expression = pExpression; }
+    void updatePrecision(TPrecisionQualifier parentPrecision);
 protected:
     TOperator flowOp;
     TIntermTyped* expression;