[clang] Run LLVM Verifier in modes without CodeGen too

Previously, the Backend_Emit{Nothing,BC,LL} modes did
not run the LLVM verifier since it is usually added via
the TargetMachine::addPassesToEmitFile method according
to the DisableVerify parameter. This is called from
EmitAssemblyHelper::AddEmitPasses, which is only relevant
for BackendAction-s that require CodeGen.

Note:
* In these particular situations the verifier is added
  to the optimization pipeline rather than the codegen
  pipeline so that it runs prior to the BC/LL emission
  pass.
* This change applies to both the old and the new PMs.
* Because the clang tests use -emit-llvm ubiquitously,
  this change will enable the verifier for them.
* A small bug is fixed in emitIFuncDefinition so that
  the clang/test/CodeGen/ifunc.c test would pass:
  the emitIFuncDefinition incorrectly passed the
  GlobalDecl of the IFunc itself to the call to
  GetOrCreateLLVMFunction for creating the resolver.

Signed-off-by: Itay Bookstein <ibookstein@gmail.com>

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D113352