[flang][driver] Re-organise the code-gen actions (nfc)

All frontend actions that generate code  (MLIR, LLVM IR/BC,
Assembly/Object Code) are re-factored as essentially one action,
`CodeGenAction`, with minor specialisations. To facilate all this,
`CodeGenAction` is extended to hold `TargetMachine` and backend action
type (MLIR vs LLVM IR vs LLVM BC vs Assembly vs Object Code).

`CodeGenAction` is no longer a pure abstract class and the
corresponding `ExecuteAction` is implemented so that it covers all use
cases. All this allows a much better code re-use.

Key functionality is extracted into some helpful hooks:
  * `SetUpTargetMachine`
  * `GetOutputStream`
  * `EmitObjectCodeHelper`
  * `EmitBCHelper`
I hope that this clarifies the overall structure. I suspect that we may
need to revisit this again as the functionality grows in complexity.

Differential Revision: https://reviews.llvm.org/D124665

1 files changed, 2 insertions, 4 deletions

diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index 8f5b313..7aef815 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -41,11 +41,9 @@ static std::unique_ptr<FrontendAction> CreateFrontendAction(
   case EmitLLVMBitcode:
     return std::make_unique<EmitLLVMBitcodeAction>();
   case EmitObj:
-    return std::make_unique<BackendAction>(
-        BackendAction::BackendActionTy::Backend_EmitObj);
+    return std::make_unique<EmitObjAction>();
   case EmitAssembly:
-    return std::make_unique<BackendAction>(
-        BackendAction::BackendActionTy::Backend_EmitAssembly);
+    return std::make_unique<EmitAssemblyAction>();
   case DebugUnparse:
     return std::make_unique<DebugUnparseAction>();
   case DebugUnparseNoSema: