[flang][driver] Add the new flang compiler and frontend drivers

Summary:

This is the first patch implementing the new Flang driver as outlined in [1],
[2] & [3]. It creates Flang driver (`flang-new`) and Flang frontend driver
(`flang-new -fc1`). These will be renamed as `flang` and `flang -fc1` once the
current Flang throwaway driver, `flang`, can be replaced with `flang-new`.

Currently only 2 options are supported: `-help` and `--version`.

`flang-new` is implemented in terms of libclangDriver, defaulting the driver
mode to `FlangMode` (added to libclangDriver in [4]). This ensures that the
driver runs in Flang mode regardless of the name of the binary inferred from
argv[0].

The design of the new Flang compiler and frontend drivers is inspired by it
counterparts in Clang [3]. Currently, the new Flang compiler and frontend
drivers re-use Clang libraries: clangBasic, clangDriver and clangFrontend.

To identify Flang options, this patch adds FlangOption/FC1Option enums.
Driver::printHelp is updated so that `flang-new` prints only Flang options.
The new Flang driver is disabled by default. To enable it, set
`-DBUILD_FLANG_NEW_DRIVER=ON` when configuring CMake and add clang to
`LLVM_ENABLE_PROJECTS` (e.g. -DLLVM_ENABLE_PROJECTS=“clang;flang;mlir”).

[1] “RFC: new Flang driver - next steps”
http://lists.llvm.org/pipermail/flang-dev/2020-July/000470.html
[2] “RFC: Adding a fortran mode to the clang driver for flang”
http://lists.llvm.org/pipermail/cfe-dev/2019-June/062669.html
[3] “RFC: refactoring libclangDriver/libclangFrontend to share with Flang”
http://lists.llvm.org/pipermail/cfe-dev/2020-July/066393.html
[4] https://reviews.llvm.org/rG6bf55804924d5a1d902925ad080b1a2b57c5c75c

co-authored-by: Andrzej Warzynski <andrzej.warzynski@arm.com>

Reviewed By: richard.barton.arm, sameeranjoshi

Differential Revision: https://reviews.llvm.org/D86089

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp b/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp
index 1d5a6c06..ff0aa6f 100644
--- a/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp
+++ b/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp
@@ -40,8 +40,8 @@ std::unique_ptr<CompilerInvocation> clang::createInvocationFromCommandLine(
   Args.push_back("-fsyntax-only");
 
   // FIXME: We shouldn't have to pass in the path info.
-  driver::Driver TheDriver(Args[0], llvm::sys::getDefaultTargetTriple(),
-                           *Diags, VFS);
+  driver::Driver TheDriver(Args[0], llvm::sys::getDefaultTargetTriple(), *Diags,
+                           "clang LLVM compiler", VFS);
 
   // Don't check that inputs exist, they may have been remapped.
   TheDriver.setCheckInputsExist(false);