Use this to define the macro location rather than the "host" dir (which
is actually for the build and not the host). Furthermore, on Windows,
the build dir is /usr/lib/swift as the host content is in the SDK.
This prepares the tests for Windows.
This could cause us to double up on the same note
for nested macro expansions, as we'd generate the
note, then when recursing back through the function
we'd compute the same note again. Also remove the
seemingly unused `lastBufferID` param.
