Document CMake support

[awarzynski]

Summary

One out-of-date section is also deleted.

[rovka]

Yay :)

flang/docs/FlangDriver.md

343

[sscalpone]

flang/docs/FlangDriver.md

337

You can make this "timeless" by restarting as something like:

Starting with CMake 3.24.0, make can detect.....

[awarzynski]

flang/docs/FlangDriver.md

337

Thanks, I like your suggestion! Sadly I've already merged this :( That's fine though - this document will require updating soon anyway (I just need to take a note to update this too).

flang/docs/FlangDriver.md

 <!--===- docs/FlangDriver.md
 
    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
 
 -->
 
 # Flang drivers
 
 ```eval_rst
 .. contents::
    :local:
 ```
 
-
-> **_NOTE:_** This document assumes that Flang's drivers can already generate code and
-> produce executables. However, this is still work-in-progress. By making this
-> assumption, we are able to prepare this document ahead-of-time and to provide
-> an overview of the design that we are working towards.
-
 There are two main drivers in Flang:
 * the compiler driver, `flang-new`
 * the frontend driver, `flang-new -fc1`
 
 > **_NOTE:_** The diagrams in this document refer to `flang` as opposed to
 > `flang-new`. This is because the diagrams reflect the final design that we
 > are still working towards. See the note on [the flang script](https://github.com/llvm/llvm-project/blob/main/flang/docs/FlangDriver.md#the-flang-script)
 > below for more context.

 `-fsyntax-only`:
 ```cpp
   case ParseSyntaxOnly:
     return std::make_unique<ParseSyntaxOnlyAction>();
 ```
 At this point you should be able to trigger that frontend action that you have
 just added using your new frontend option.
 
+
+# CMake Support
+As of [#7246](https://gitlab.kitware.com/cmake/cmake/-/merge_requests/7246)
+(and soon to be released CMake 3.24.0), `cmake` can detect `flang-new` as a
+supported Fortran compiler. You can configure your CMake projects to use
+`flang-new` as follows:
+```bash
+cmake -DCMAKE_Fortran_FLAGS="-flang-experimental-exec" -DCMAKE_Fortran_COMPILER=<path/to/flang-new> <src/dir>
+```
+You should see the following in the output:
+```
+-- The Fortran compiler identification is LLVMFlang <version>
+```
+where `<version>` corresponds to the LLVM Flang version. Note that while
+generating executables remains experimental, you will need to inform CMake to
+use the `-flang-experimental-exec` flag when invoking `flang-new` as in the
+example above.
+
 # Testing
 In LIT, we define two variables that you can use to invoke Flang's drivers:
 * `%flang` is expanded as `flang-new` (i.e. the compiler driver)
 * `%flang_fc1` is expanded as `flang-new -fc1` (i.e. the frontend driver)
 
 For most regression tests for the frontend, you will want to use `%flang_fc1`.
 In some cases, the observable behaviour will be identical regardless of whether
 `%flang` or `%flang_fc1` is used. However, when you are using `%flang` instead