[EHStreamer] Omit @LPStart when function has no landing pads

When no landing pads exist for a function, `@LPStart` is undefined and must be omitted.

EH table is generally not emitted for functions without landing pads, except when the personality function is uknown (`!isNoOpWithoutInvoke(classifyEHPersonality(Per))`). In that case, we must omit `@LPStart` even when machine function splitting is enabled.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D131626