Add checks about references to non-lvalues.

Both ReturnStmt and DeclStmt now check the values being associated
with references to make sure that they are legal (e.g. it's illegal
to assign a varying lvalue, or a compile-time constant to a reference
type).  Previously we didn't catch this and would end up hitting
assertions in LLVM when code did this stuff.

Mostly fixes issue #225 (except for adding a FAQ about what this
error message means.)

tests_errors/return-ref-1.ispc

@@ -0,0 +1,5 @@
+// Illegal to return non-lvalue from function returning reference type
+
+float &func(uniform float a[], int i, float f) {
+    return 1.f;
+}