[Macros] Remove ModuleDecl::isInGeneratedBuffer.
This method was misleading. The majority of callers (all but one!) don't want
to unconditionally treat all locations in any macro expansion buffer the
same way, because the code also must handle nested macro expansions. There
is one part of SourceKit (and possibly others) that really do want to ignore
all macro expansions, but those can be handled within SourceKit / IDE code,
because I don't believe this utility is useful in the frontend.
(cherry picked from commit b958e4352869d4ff11f797c4059526c1e8f20853)
