SPV: Partially address #2293: correct "const in" precision matching.

Track whether formal parameters declare reduced precision and match
that with arguments, and if they differ, make a copy to promote the
precision.

Test/spv.precisionArgs.frag

@@ -0,0 +1,15 @@
+#version 310 es
+
+precision mediump float;
+
+void fooConst(const in float f, const in highp float g)
+{
+}
+
+void main()
+{
+    float aM, bM;
+    highp float aH, bH;
+    fooConst(aM, bM);   // must copy bM
+    fooConst(aH, bH);   // must copy aH
+}