[LTO][MC] Discard non-prevailing defined symbols in module-level assembly

This is the alternative approach to D96931.

In LTO, for each module with inlineasm block, prepend directive ".lto_discard <sym>, <sym>*" to the beginning of the inline
asm.  ".lto_discard" is both a module inlineasm block marker and (optionally) provides a list of symbols to be discarded.

In MC while emitting for inlineasm, discard symbol binding & symbol
definitions according to ".lto_disard".

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D98762