Add -Wstatic-local-in-inline, which warns about using a static local
variable in a C99 inline (but not static-inline or extern-inline)
function definition.

The standard doesn't actually say that this doesn't apply to
"extern inline" definitions, but that seems like a useful extension,
and it at least doesn't have the obvious flaw that a static
mutable variable in an externally-available definition does.

rdar://13535367