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

This patch adds support for:
  * `-S` in Flang's compiler and frontend drivers,
and implements:
  * `-emit-obj` in Flang's frontend driver and `-c` in Flang's compiler
    driver (this is consistent with Clang).
(these options were already available before, but only as placeholders).
The semantics of these options in Clang and Flang are identical.

The `EmitObjAction` frontend action is renamed as `BackendAction`. This
new name more accurately reflects the fact that this action will
primarily run the code-gen/backend pipeline in LLVM. It also makes more
sense as an action implementing both `-emit-obj` and `-S` (originally,
it was just `-emit-obj`).

`tripleName` from FirContext.cpp is deleted and, when a target triple is
required, `mlir::LLVM::LLVMDialect::getTargetTripleAttrName()` is used
instead. In practice, this means that `fir.triple` is replaced with
`llvm.target_triple`. The former was effectively ignored. The latter is
used when lowering from the LLVM dialect in MLIR to LLVM IR (i.e. it's
embedded in the generated LLVM IR module). The driver can then re-use
it when configuring the backend. With this change, the LLVM IR files
generated by e.g. `tco` will from now on contain the correct target
triple.

The code-gen.f90 test is replaced with code-gen-x86.f90 and
code-gen-aarch64.f90. With 2 seperate files we can verify that
`--target` is correctly taken into account. LIT configuration is updated
to enable e.g.:
```
! REQUIRES: aarch64-registered-target
```

Differential Revision: https://reviews.llvm.org/D120568

1 files changed, 5 insertions, 1 deletions

diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index d97833b..4e8c225 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -38,7 +38,11 @@ static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
   case EmitLLVM:
     return std::make_unique<EmitLLVMAction>();
   case EmitObj:
-    return std::make_unique<EmitObjAction>();
+    return std::make_unique<BackendAction>(
+        BackendAction::BackendActionTy::Backend_EmitObj);
+  case EmitAssembly:
+    return std::make_unique<BackendAction>(
+        BackendAction::BackendActionTy::Backend_EmitAssembly);
   case DebugUnparse:
     return std::make_unique<DebugUnparseAction>();
   case DebugUnparseNoSema: