servo: Merge #19701 - Update Arch Linux setup to include clang (from ntenczar:update-readme-clang); r=jdm

<!-- Please describe your changes on the following line: -->
`clang` is needed to compile servo, but isn't listed as one of the dependencies needed for arch and isn't part of the `base-devel` group.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X]  no applicable github issue

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's a README change

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Source-Repo: https://github.com/servo/servo
Source-Revision: dd27fbcb3f31ae7a72d5d3ba5d22db318249acbd

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 52f76145122ae13d8154f1306d850fcbc35bf9f3

servo/README.md

@@ -107,7 +107,7 @@ sudo zypper install libX11-devel libexpat-devel libbz2-devel Mesa-libEGL-devel M
 #### On Arch Linux
 
 ``` sh
-sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip mesa cmake bzip2 libxmu glu pkg-config
+sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip mesa cmake bzip2 libxmu glu pkg-config clang
 ```
 #### On Gentoo Linux