Sema: Try to bind extensions without expanding macros
Macro expansion can call typeCheckExpr(), which performs qualified
lookups. So if we expand macros while binding extensions, these
qualified lookups can fail because they cannot find members of
extensions that have not been bound yet.
To fix this, try binding extensions without performing macro
expansion first. If any extensions remain at the end, we fall back
to the old behavior, and try to bind them again, this time
performing macro expansion.
Fixes rdar://149798059.
