omit prefix data for async functions sometimes

When `@frameSize` is never called, and `@asyncCall` on a runtime-known
pointer is never used, no prefix data for async functions is needed.

Related: #3160
d74b8567cfAndrew Kelley committed on 9/3/2019, 1:31:26 AMparent00d82e3
3 files changedLine totals unavailable