clang docs: Update page a bit.

The Windows bits were outdated.

Bug: none
Change-Id: I45fd6b70e013491080bc7028b7ce5be600e8fab2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790124
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694325}

docs/clang.md

 # Clang
 
-[Clang](http://clang.llvm.org/) is the main supported compiler when
-building Chromium on all platforms.
+Chromium ships a prebuilt [clang](http://clang.llvm.org) binary.
+It's just upstream clang built at a known-good revision that we
+bump every two weeks or so.
 
-Known [clang bugs and feature
-requests](http://code.google.com/p/chromium/issues/list?q=label:clang).
+This is the only supported compiler for building Chromium.
 
 [TOC]
 
-## Building with clang
+## Using gcc on Linux
 
-This happens by default, with clang binaries being fetched by gclient
-during the `gclient runhooks` phase. To fetch them manually, or build
-a local custom clang, use
-
-    tools/clang/scripts/update.py
-
-Run `gn args` and make sure there is no `is_clang = false` in your args.gn file.
-
-Build: `ninja -C out/gn chrome`
-
-## Reverting to gcc on Linux or MSVC on Windows
-
-There are no bots that test this but `is_clang = false` will revert to
-gcc on Linux and to Visual Studio on Windows. There is no guarantee it
-will work.
+`is_clang = false` will make the build use system gcc on Linux. There are no
+bots that test this and there is no guarantee it will work, but we accept
+patches for this configuration.
 
 ## Mailing List
 
@@ -66,16 +54,8 @@ See [clang_static_analyzer.md](clang_static_analyzer.md).
 
 ## Windows
 
-Since October 2017, clang is the default compiler on Windows. It uses
-MSVC's linker and SDK, so you still need to have Visual Studio with
-C++ support installed.
-
-To use MSVC's compiler (if it still works), use `is_clang = false`.
-
-Current brokenness:
-
-*   To get colored diagnostics, you need to be running
-    [ansicon](https://github.com/adoxa/ansicon/releases).
+clang is the default compiler on Windows. It uses MSVC's SDK, so you still need
+to have Visual Studio with C++ support installed.
 
 ## Using a custom clang binary