Thew installation instructions are now

    “brew install llvm”

before building Qt.

LLVM_INSTALL_DIR can still be set to override the
llvm location.

Task-number: QTBUG-65762
Task-number: QTBUG-66353
Change-Id: I9a17d0c280aa6b8a8b8117b8a5c7121dcfe906c6
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>