[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

1 files changed, 2 insertions, 0 deletions

diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index 0315fd5..50c9fca 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -34,6 +34,8 @@ static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
     break;
   case ParseSyntaxOnly:
     return std::make_unique<ParseSyntaxOnlyAction>();
+  case EmitObj:
+    return std::make_unique<EmitObjAction>();
     break;
   default:
     break;