[ELF] Assert on invalid GOT or PLT relocations

Because of https://llvm.org/PR50675, we can end up producing a PLT
relocation referencing a symbol that's dropped from the dynamic symbol
table, which in turn causes a crash at runtime. We ran into this again
recently, resulting in crashes for our users. A subsequent diff will fix
that issue, but add an assert to catch it if it happens again.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D123985