* [flang][driver] Extend support for LLVM IR/BC files

Before this change, `clangDriver` (i.e. the compiles driver library)
would always use `clang` to compile LLVM IR or LLVM BC files (even when
`flang-new` was used to invoke the driver). With this change, Flang will
finally accept LLVM IR and LLVM BC alongside e.g. Fortran input files.
This means that from now on it is up to the user to decide whether to
use Flang or Clang to compile an LLVM file (`flang-new -c file.bc` and
`clang -c file.bc`, respectively).

To complement this, support for generating LLVM BC files is added. This
requires the `BitcodeWriterPass` pass to be run on the input LLVM IR
module and is implemented as a dedicated frontend aciton.

The new functionality as seen by the user:
1. LLVM BC file generation (`flang-new -c -emit-llvm file.90` or
   `flang-new -fc1 -emit-llvm-bc file.f90`)
2. Ability to consume LLVM IR and BC files and to lower them to object
   files (`flang-new -c file.bc` or `flang-new -c file.ll`)

The new behaviour is consistent with `clang` and `clang -cc1`.

* [flang][driver] Add support for `-save-temps`

This patch adds support for `-save-temps` in `flang-new`, Flang's
compiler driver. The semantics of this option are inherited from Clang.

The file extension for temporary Fortran preprocessed files is set to
`i`. This is identical to what Clang uses for C (or C++) preprocessed
files. I have tried researching what other compilers do here, but I
couldn't find any definitive answers. One GFortran thread [1] suggests
that indeed it is not clear what the right approach should be.

As `-save-temps` makes the compiler run every phase separately, in Flang
one currently has to use it with `-fno-integrated-as`. Otherwise, an
invocation to the integrated assembler driver is generated (i.e.
something equivalent to `clang -cc1as` from Clang), but this is not yet
implemented in Flang.

[1] https://gcc.gnu.org/bugzilla//show_bug.cgi?id=81615

* Add missing llvm-dis dependency

…lvm#1471)

This reverts commit 5595357.