[compiler] fix false positive "mutate frozen" validation with refs

The test case here previously reported a "Cannot modify local variables after render completes" error (from ValidateNoFreezingKnownMutableFunctions). This happens because one of the functions passed to a hook clearly mutates a ref — except that we try to ignore mutations of refs! The problem in this case is that the `const ref = ...` was getting converted to a context variable since the ref is accessed in a function before its declaration. We don't infer types for context variables at all, and our ref handling is based on types, so we failed to ignore this ref mutation.

The fix is to recognize that `StoreLocal const ...` is a special case: the variable may be referenced in code before the declaration, but at runtime it's either a TDZ error or the variable will have the type from the declaration. So we can safely infer a type.