Reading between the lines of the spec, Functions are also scopes.

Move common scope fields into the base Scope class.

Fix `scope` fields requiring a `LexicalBlock` to take the more general `Scope`.

Bug: b/148401179
Change-Id: Iac7b4fd4440a701905e2dbcdf71b91146ea8ff7d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42248Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>