* Prevent ICE when formatting item-only `vec!{}`
Fixes 5735
Attempting to format invocations of macros which are considered "forced
bracket macros" (currently only `vec!`), but are invoked with braces
instead of brackets, and contain only items in their token trees,
currently triggers an ICE in rustfmt. This is because the function that
handles formatting macro invocations containing only items,
`rewrite_macro_with_items`, assumes that the forced delimiter style of
the macro being formatted is the same as the delimiter style in the
macro's source text when attempting to locate the span after the macro's
opening delimiter. This leads to the construction of an invalid span,
triggering the ICE.
The fix here is to pass the old delimiter style to
`rewrite_macro_with_items` as well, so that it can successfully locate
the span.
