SI-8013 Nowarn on macro str interpolation

When warning about stray "foo $bar" under `-Xlint`,
which may be missing an interpolator id, suppress
the warning if we're in the middle of a macro
expansion, since we have no useful heuristic to
apply to the expanded tree.

The test for whether the string is part of an
expanded tree is to check all open macros for
an expanded tree that contains the literal tree
under scrutiny. (This is deemed more paranoid
than looking for a macro application that is an
enclosing position.)

Review by @retronym