This fixes a recent regression introduced in the changes to make
TRC construction lazier in b33a71d64f.
Calling VarDecl::hasInitialValue() can ask for attached property
wrappers, which can trigger macro expansion. However, macro
expansion can result in TRC construction, causing a cycle.
Instead, just always build a lazy TRC for the initializer. It
might not be needed, but it's safest to not ask anything of the
VarDecl at all.
Fixes rdar://118452948.
Add a test case for Observable types that are extended from other
source files. Prior to the recent changes to make
`TypeRefinementContext` more lazy, this would trigger circular
references through the `TypeRefinementContextBuilder`.
Finishes rdar://112079160.
