[flang][driver] Extend the flang bash script to act as a driver

Until now, `f18` would:
  1. Use Flang to unparse the input files
  2. Call an external Fortran compiler to compile the unparsed source
  files (generated in step 1)

With this patch, `f18` will stop after unparsing the input source files,
i.e. step 1 above. The `flang` bash script will take care of step 2,
i.e. calling an external Fortran compiler driver to compile them. This
way:
  * the functionality of `f18` is reduced - it will only drive Flang (as
  opposed to delegating code-generation to an external tool on top of
  this)
  * we will able to switch between `f18` and `flang-new` for unparsing before
  an external Fortran compiler is called for code-generation

The updated `flang` bash script needs to specify the output file when
using the `-fdebug-unparse` action. Both `f18` and `flang-new` have been
updated accordingly.

These changes were discussed in [1] as a requirement for replacing `f18`
with `flang-new`.

[1] https://lists.llvm.org/pipermail/flang-dev/2021-April/000677.html

Differential Revision: https://reviews.llvm.org/D103177

flang/lib/Frontend/FrontendActions.cpp

@@ -257,8 +257,12 @@ void DebugUnparseAction::ExecuteAction() {
   auto &invoc = this->instance().invocation();
   auto &parseTree{instance().parsing().parseTree()};
 
+  CompilerInstance &ci = this->instance();
+  auto os{ci.CreateDefaultOutputFile(
+      /*Binary=*/false, /*InFile=*/GetCurrentFileOrBufferName())};
+
   // TODO: Options should come from CompilerInvocation
-  Unparse(llvm::outs(), *parseTree,
+  Unparse(*os, *parseTree,
       /*encoding=*/Fortran::parser::Encoding::UTF_8,
       /*capitalizeKeywords=*/true, /*backslashEscapes=*/false,
       /*preStatement=*/nullptr,

flang/tools/f18/CMakeLists.txt

@@ -62,10 +62,13 @@ add_custom_target(module_files ALL DEPENDS ${MODULE_FILES})
 
 install(TARGETS f18 DESTINATION bin)
 
+set(FLANG_DEFAULT_DRIVER "flang-new")
+if (NOT FLANG_BUILD_NEW_DRIVER)
+  set(FLANG_DEFAULT_DRIVER "f18")
+endif()
+
 # This flang shell script will only work in a POSIX shell.
 if (NOT WIN32)
-  file(COPY ${CMAKE_CURRENT_SOURCE_DIR}/flang
-    DESTINATION ${CMAKE_BINARY_DIR}/bin
-    FILE_PERMISSIONS OWNER_EXECUTE OWNER_READ OWNER_WRITE)
+  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/flang.in ${CMAKE_BINARY_DIR}/bin/flang @ONLY)
   install(PROGRAMS ${CMAKE_BINARY_DIR}/bin/flang DESTINATION bin)
 endif()

flang/tools/f18/f18.cpp

@@ -319,7 +319,22 @@ std::string CompileFortran(std::string path, Fortran::parser::Options options,
     Fortran::parser::DumpTree(llvm::outs(), parseTree, &asFortran);
   }
   if (driver.dumpUnparse) {
-    Unparse(llvm::outs(), parseTree, driver.encoding, true /*capitalize*/,
+    // Prepare the output stream
+    std::unique_ptr<llvm::raw_fd_ostream> os;
+    std::string outputFile = "-";
+    if (!driver.outputPath.empty()) {
+      outputFile = driver.outputPath;
+    }
+
+    std::error_code EC;
+    os.reset(new llvm::raw_fd_ostream(
+        outputFile, EC, llvm::sys::fs::OF_TextWithCRLF));
+    if (EC) {
+      llvm::errs() << EC.message() << "\n";
+      std::exit(EXIT_FAILURE);
+    }
+
+    Unparse(*os, parseTree, driver.encoding, true /*capitalize*/,
         options.features.IsEnabled(
             Fortran::common::LanguageFeature::BackslashEscapes),
         nullptr /* action before each statement */,