[Flang] Add support to use LTO specific pipelines

Thin and full LTO modes use different pre-link pipelines compared to
regular compilation. This patch adds support for calling those pipelines.

This patch closely mimics Clang's implementation with the exception that I
changed the codegen option name from `PrepareForLTO` to `PrepareForFullLTO`
to be more precise.

With this patch:
  - Compilation for full LTO should be as we expect (except possibly
  missing optimizations enabled by module summaries which we do not
  produce yet).
  - thinLTO uses the correct prelink pipeline but will use the postlink
  backend for fullLTO due to missing metadata and summary in the llvm
  module. I have added a warning regarding this: `flang-new: warning: the
  option '-flto=thin' is a work in progress`.

Differential Revision: https://reviews.llvm.org/D142420

Change-Id: I6b94b775b5b8e93340e520c5cd4bf60834b2e209