Mozilla's build machines are currently applying this patch locally, but I thought I'd offer it upstream because it should be pretty harmless.
clang-cl has some sanity checks to make sure that the cl.exe it finds is actually the Microsoft compiler and not something else. The code expects the path to look like:
It doesn't work on Mozilla's build automation where we have a fake Visual Studio installation that looks like: c:\vs2017\VC\bin\HostX64\x64\cl.exe
This patch reduces the path requirement to ...\bin\Host*\*\cl.exe.