This started out as a crash, where an expression macro could not be
defined in terms of one of the builtin macros (e.g., `#line`), because
we were expecting a macro expansion expression but didn't get one.
Easy fix.
However, this uncovered a second bug, which is that we couldn't handle
an expression macro expansino to `#line`. This is because we were
parsing the macro expansion buffer as "top level items", which treats
`#line` at the start of a line as a deprecated alias of
`#sourceLocation`. Switch over to parsing a single expression in these
contexts, and fix up an issue where `#isolation` didn't even have that
expression.
Fixes rdar://139372780.
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.
