[ORC][C-bindings] Fix some ORC C bindings function names and signatures.

LLVMOrcDisposeObjectLayer and LLVMOrcExecutionSessionGetJITDylibByName did not
have matching signatures between the C-API header and binding implementations.
Fixes http://llvm.org/PR49745.

Patch by Mats Larsen. Thanks Mats!

Reviewed by: lhames

Differential Revision: https://reviews.llvm.org/D99478