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.)

type.h

@@ -828,4 +828,8 @@ private:
     const std::vector<SourcePos> paramPositions;
 };
 
+inline bool IsReferenceType(const Type *t) {
+    return dynamic_cast<const ReferenceType *>(t) != NULL;
+}
+
 #endif // ISPC_TYPE_H