Parse expression macro expansion buffers as expressions
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.
