Revert "[flang] Fix seg fault `CodeGenAction::executeAction()` (#78269)" (#78667)

This reverts commit 99cae9a44fca4cfbd6ee82f196051cbdf6571fa1.

Temporarily until I reproduce and fix a linker issue:
```
FAILED: tools/flang/unittests/Frontend/FlangFrontendTests
...
/usr/bin/ld: tools/flang/unittests/Frontend/CMakeFiles/FlangFrontendTests.dir/CodeGenActionTest.cpp.o: undefined reference to symbol '_ZN4llvm11LLVMContextC1Ev'
/usr/bin/ld: /work1/omp-nightly/build/git/trunk18.0/build/llvm-project/lib/libLLVMCore.so.18git: error adding symbols: DSO missing from command line
```

1 files changed, 0 insertions, 109 deletions

diff --git a/flang/unittests/Frontend/CodeGenActionTest.cpp b/flang/unittests/Frontend/CodeGenActionTest.cpp
deleted file mode 100644
index 9d798c7..0000000
--- a/flang/unittests/Frontend/CodeGenActionTest.cpp
+++ /dev/null
@@ -1,109 +0,0 @@
-//===- unittests/Frontend/CodeGenActionTest.cpp --- FrontendAction tests --===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-//
-// Unit tests for CodeGenAction.
-//
-//===----------------------------------------------------------------------===//
-
-#include "mlir/IR/Builders.h"
-#include "flang/Frontend/CompilerInstance.h"
-#include "flang/Frontend/FrontendActions.h"
-#include "flang/Frontend/TextDiagnosticPrinter.h"
-
-#include "gtest/gtest.h"
-
-#include <memory>
-
-using namespace Fortran::frontend;
-
-namespace test {
-class DummyDialect : public ::mlir::Dialect {
-  explicit DummyDialect(::mlir::MLIRContext *context)
-      : ::mlir::Dialect(getDialectNamespace(), context,
-            ::mlir::TypeID::get<DummyDialect>()) {
-    initialize();
-  }
-
-  void initialize();
-  friend class ::mlir::MLIRContext;
-
-public:
-  ~DummyDialect() override = default;
-  static constexpr ::llvm::StringLiteral getDialectNamespace() {
-    return ::llvm::StringLiteral("dummy");
-  }
-};
-
-namespace dummy {
-class FakeOp : public ::mlir::Op<FakeOp> {
-public:
-  using Op::Op;
-
-  static llvm::StringRef getOperationName() { return "dummy.fake"; }
-
-  static ::llvm::ArrayRef<::llvm::StringRef> getAttributeNames() { return {}; }
-
-  static void build(
-      ::mlir::OpBuilder &odsBuilder, ::mlir::OperationState &odsState) {}
-};
-} // namespace dummy
-} // namespace test
-
-MLIR_DECLARE_EXPLICIT_TYPE_ID(::test::DummyDialect)
-MLIR_DEFINE_EXPLICIT_TYPE_ID(::test::DummyDialect)
-
-namespace test {
-
-void DummyDialect::initialize() { addOperations<::test::dummy::FakeOp>(); }
-} // namespace test
-
-// A test CodeGenAction to verify that we gracefully handle failure to convert
-// from MLIR to LLVM IR.
-class LLVMConversionFailureCodeGenAction : public CodeGenAction {
-public:
-  LLVMConversionFailureCodeGenAction()
-      : CodeGenAction(BackendActionTy::Backend_EmitLL) {
-    mlirCtx = std::make_unique<mlir::MLIRContext>();
-    mlirCtx->loadDialect<test::DummyDialect>();
-
-    mlir::Location loc(mlir::UnknownLoc::get(mlirCtx.get()));
-    mlirModule =
-        std::make_unique<mlir::ModuleOp>(mlir::ModuleOp::create(loc, "mod"));
-
-    mlir::OpBuilder builder(mlirCtx.get());
-    builder.setInsertionPointToStart(&mlirModule->getRegion().front());
-    // Create a fake op to trip conversion to LLVM.
-    builder.create<test::dummy::FakeOp>(loc);
-
-    llvmCtx = std::make_unique<llvm::LLVMContext>();
-  }
-};
-
-TEST(CodeGenAction, GracefullyHandleLLVMConversionFailure) {
-  std::string diagnosticOutput;
-  llvm::raw_string_ostream diagnosticsOS(diagnosticOutput);
-  auto diagPrinter = std::make_unique<Fortran::frontend::TextDiagnosticPrinter>(
-      diagnosticsOS, new clang::DiagnosticOptions());
-
-  CompilerInstance ci;
-  ci.createDiagnostics(diagPrinter.get(), /*ShouldOwnClient=*/false);
-  ci.setInvocation(std::make_shared<CompilerInvocation>());
-  ci.setOutputStream(std::make_unique<llvm::raw_null_ostream>());
-  ci.getInvocation().getCodeGenOpts().OptimizationLevel = 0;
-
-  FrontendInputFile file("/dev/null", InputKind());
-
-  LLVMConversionFailureCodeGenAction action;
-  action.setInstance(&ci);
-  action.setCurrentInput(file);
-
-  consumeError(action.execute());
-  ASSERT_EQ(diagnosticsOS.str(),
-      "error: Lowering to LLVM IR failed\n"
-      "error: failed to create the LLVM module\n");
-}