[flang][driver] Delete `f18` (i.e. the old Flang driver)

This patch removes `f18`, a.k.a. the old driver. It is being replaced
with the new driver, `flang-new`, which has reached feature parity with

[flang][driver] Delete `f18` (i.e. the old Flang driver)

This patch removes `f18`, a.k.a. the old driver. It is being replaced
with the new driver, `flang-new`, which has reached feature parity with
`f18` a while ago. This was discussed in [1] and also in [2].

With this change, `FLANG_BUILD_NEW_DRIVER` is no longer needed and is
also deleted. This means that we are making the dependency on Clang permanent
(i.e. it cannot be disabled with a CMake flag).

LIT set-up is updated accordingly. All references to `f18` or `f18.cpp`
are either updated or removed.

The `F18_FC` variable from the `flang` bash script is replaced with
`FLANG_FC`. The former is still supported for backwards compatibility.

[1] https://lists.llvm.org/pipermail/flang-dev/2021-June/000742.html
[2] https://reviews.llvm.org/D103177

Differential Revision: https://reviews.llvm.org/D105811

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[flang] Add -fsyntax-only to f18; retain -fparse-only synonym

Now that semantics is working, the standard -fsyntax-only option of
GNU and Clang should be used as the name of the option that causes
f

[flang] Add -fsyntax-only to f18; retain -fparse-only synonym

Now that semantics is working, the standard -fsyntax-only option of
GNU and Clang should be used as the name of the option that causes
f18 to just run the front-end. Support both options in f18, silently
accepting the old option as a synonym for the new one (as
preferred by the code owner), and replace all instances of the
old -fparse-only option with -fsyntax-only throughout the source base.

Differential Revision: https://reviews.llvm.org/D95887

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[flang] Add new documentation main page

Add a new index page to be the Flang documentation mainpage instead of
Overview.md, which jumps straight into the compiler Design. The index file
needs to be

[flang] Add new documentation main page

Add a new index page to be the Flang documentation mainpage instead of
Overview.md, which jumps straight into the compiler Design. The index file
needs to be in .rst format to use the toctree directive to create table of
contents.

Also use the sphinx_markdown_tables extension to generate html tables form
markdown.

A number of additional style changes to the existing docs were needed to make
this work well:
* Convert all headings to the # style, which works better with toctree's
titlesonly option. Ensure that there is only one top-level heading per
document.
* Add a title to documents that don't have one for rendering on the index.
* Convert the grammar docs from .txt to .md. for better rendering
* Fixed broken link to a section in another document - sphinx does not seem to
support anchor links in markdown files.

Depends on D87226

Reviewed By: sameeranjoshi

Differential Revision: https://reviews.llvm.org/D87242

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Revert D86875 "[Flang][NFC] Remove license comments from files in docs/ folder."

This reverts commit f787c9a90c69f, this was causing some build issues.

[Flang][NFC] Remove license comments from files in docs/ folder.

Solves issue https://reviews.llvm.org/D86131#2247275

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D86875

[Flang] Move markdown files(.MD) from documentation/ to docs/

Summary:
Other LLVM sub-projects use docs/ folder for documentation files.
Follow LLVM project policy.
Modify `documentation/` reference

[Flang] Move markdown files(.MD) from documentation/ to docs/

Summary:
Other LLVM sub-projects use docs/ folder for documentation files.
Follow LLVM project policy.
Modify `documentation/` references in sources to `docs/`.
This patch doesn't modify files to reStructuredText(.rst) file format.

Reviewed By: DavidTruby, sscalpone

Differential Revision: https://reviews.llvm.org/D85884

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