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[flang][driver] Add support for `-c` and `-emit-obj`

Authored by awarzynski on Jan 7 2021, 1:08 AM.

Description

[flang][driver] Add support for -c and -emit-obj

This patch adds a frontend action for emitting object files. While Flang
does not support code-generation, this action remains a placeholder.
This patch simply provides glue-code to connect the compiler driver
with the appropriate frontend action.

The new action is triggered with the -c compiler driver flag, i.e.
flang-new -c. This is then translated to flang-new -fc1 -emit-obj,
so -emit-obj has to be marked as supported as well.

As code-generation is not available yet, flang-new -c results in a
driver error:

error: code-generation is not available yet

Hopefully this will help communicating the level of available
functionality within Flang.

The definition of emit-obj is updated so that it can be shared between
Clang and Flang. As the original definition was enclosed within a
Clang-specific TableGen let statement, it is extracted into a new let
statement. That felt like the cleanest option.

I also commented out -triple in Flang::ConstructJob and updated some
comments there. This is similar to https://reviews.llvm.org/D93027. I
wanted to make sure that it's clear that we can't support -triple
until we have code-generation. However, once code-generation is
available we _will need_ -triple.

As this patch adds -emit-obj, the emit-obj.f90 becomes irrelevant and
is deleted. Instead, phases.f90 is added to demonstrate that users can
control compilation phases (indeed, -c is a phase control flag).

Reviewed By: SouraVX, clementval

Differential Revision: https://reviews.llvm.org/D93301