[flang][driver] NFC: Make code more in line with LLVM style

This patch basically implements [1] in ExecuteCompilerInvocation.cpp. It
also:
  * replaces `CreateFrontendBaseAction` with `CreateFrontendAction`
    (only one method is needed ATM, this change removes the extra
    indirection)
  * removes `InvalidAction` from the `ActionKind` enum (I don't think it
    adds much and keeping it would mean adding a new void case in
    `CreateFrontendAction`)
  * sets the default frontend action in FrontendOptions.h to
    `ParseSyntaxOnly` (note that this is still overridden independently
    in `ParseFrontendArg` in CompilerInvocation.cpp)

No new functionality is added, hence no tests.

[1] https://llvm.org/docs/CodingStandards.html#don-t-use-default-labels-in-fully-covered-switches-over-enumerations

Differential Revision: https://reviews.llvm.org/D124245

flang/include/flang/Frontend/FrontendOptions.h

@@ -20,8 +20,6 @@
 namespace Fortran::frontend {
 
 enum ActionKind {
-  InvalidAction = 0,
-
   /// -test-io mode
   InputOutputTest,
 
@@ -244,7 +242,7 @@ struct FrontendOptions {
   std::string outputFile;
 
   /// The frontend action to perform.
-  frontend::ActionKind programAction;
+  frontend::ActionKind programAction = ParseSyntaxOnly;
 
   // The form to process files in, if specified.
   FortranForm fortranForm = FortranForm::Unknown;

flang/include/flang/FrontendTool/Utils.h

@@ -17,13 +17,6 @@
 namespace Fortran::frontend {
 
 class CompilerInstance;
-class FrontendAction;
-
-/// Construct the FrontendAction of a compiler invocation based on the
-/// options specified for the compiler invocation.
-///
-/// \return - The created FrontendAction object
-std::unique_ptr<FrontendAction> CreateFrontendAction(CompilerInstance &ci);
 
 /// ExecuteCompilerInvocation - Execute the given actions described by the
 /// compiler invocation object in the given compiler instance.

flang/lib/Frontend/CompilerInvocation.cpp

@@ -98,9 +98,6 @@ static void ParseTargetArgs(TargetOptions &opts, llvm::opt::ArgList &args) {
 
 // Tweak the frontend configuration based on the frontend action
 static void setUpFrontendBasedOnAction(FrontendOptions &opts) {
-  assert(opts.programAction != Fortran::frontend::InvalidAction &&
-      "Fortran frontend action not set!");
-
   if (opts.programAction == DebugDumpParsingLog)
     opts.instrumentedParse = true;
 

flang/lib/Frontend/FrontendAction.cpp

@@ -11,7 +11,6 @@
 #include "flang/Frontend/FrontendActions.h"
 #include "flang/Frontend/FrontendOptions.h"
 #include "flang/Frontend/FrontendPluginRegistry.h"
-#include "flang/FrontendTool/Utils.h"
 #include "clang/Basic/DiagnosticFrontend.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/VirtualFileSystem.h"

flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp

@@ -24,11 +24,10 @@
 
 namespace Fortran::frontend {
 
-static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
+static std::unique_ptr<FrontendAction> CreateFrontendAction(
     CompilerInstance &ci) {
 
-  ActionKind ak = ci.frontendOpts().programAction;
-  switch (ak) {
+  switch (ci.frontendOpts().programAction) {
   case InputOutputTest:
     return std::make_unique<InputOutputTestAction>();
   case PrintPreprocessedInput:
@@ -90,25 +89,9 @@ static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
     ci.diagnostics().Report(diagID) << ci.frontendOpts().ActionName;
     return nullptr;
   }
-  default:
-    break;
-    // TODO:
-    // case ParserSyntaxOnly:
-    // case EmitLLVM:
-    // case EmitLLVMOnly:
-    // case EmitCodeGenOnly:
-    // (...)
   }
-  return 0;
-}
 
-std::unique_ptr<FrontendAction> CreateFrontendAction(CompilerInstance &ci) {
-  // Create the underlying action.
-  std::unique_ptr<FrontendAction> act = CreateFrontendBaseAction(ci);
-  if (!act)
-    return nullptr;
-
-  return act;
+  llvm_unreachable("Invalid program action!");
 }
 
 bool ExecuteCompilerInvocation(CompilerInstance *flang) {