The failing test is HookTest.IATHook. I suspect the browser_tests fail for the same reason, and the IAT test will be the smaller thing to look at. The browser tests crash with this on the stack:
base::win::IATPatchFunction::PatchFromModule

Last green: https://build.chromium.org/p/chromium.fyi/builders/CrWinClangLLD64%20tester/builds/1843
First red: https://build.chromium.org/p/chromium.fyi/builders/CrWinClangLLD64%20tester/builds/1846

Good LLVM revision: 303157
Bad LLVM revision: 303527

This is basically the same window as  http://crbug.com/728726 , but I doubt it's the .def file parser change because this is x64, which doesn't have the same stdcall mangling issues that 32-bit x86 does.

I suspect this is LLD r303374, which merged the IAT and ILT.