[Clang] Added options for integrated backend.

Following the new flow for external object code emission,
provide flags to switch between integrated and external
backend similar to the integrated assembler options.

SPIR-V target is the only user of this functionality at
this point.

This patch also updated SPIR-V documentation to clarify
that integrated object code emission for SPIR-V is an
experimental feature.

Differential Revision: https://reviews.llvm.org/D125679