Auto merge of #19701 - ntenczar:update-readme-clang, r=jdm

Update Arch Linux setup to include clang

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`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.

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- [X]  no applicable github issue

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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