[NFC] Make `DeclContext::noload_lookup()` accept transparent context

Now the `DeclContext::noload_lookup()` asserts that 'this' is not a
transparent context. However, this is not consistent with
`DeclContext::lookup()`, which will lookup into its parent context if
'this' is a transparent context.

This patch makes the behavior of `DeclContext::noload_lookup()` to be
consistent with `DeclContext::lookup()`, to lookup into the parent
context if 'this' is a transparent context.