LLVM 12 (not yet released, currently at RC3) will be required for building Chromium M90+ (currently in the beta channel; stable release planned for 2021-04-13; s. 5e42fea).
cc @ggreif: I couldn't find a draft PR for LLVM 12 yet. Are you already working on it or currently busy?

LLVM Release Schedule:

• 12.0.0:
  • Jan 26: release/12.x branch, 12.0.0-rc1
  • Feb 23: 12.0.0-rc2
  • Mar 2: 12.0.0-final