llvm_tools: add aggregate board root canonicalization

Many error messages fron boards will talk about absolute paths, like
/build/atlas/foo/bar/baz.cc. When aggregating errors across many boards,
this can lead to the same warning appearing quite often at paths that
only differ in the board's name.

Allowing translation of `/build/atlas` to `/build/{board}` makes this
easier to deal with.

BUG=b:316172255
TEST=Unittests

Change-Id: I0312b782b6aa4754034236605d1311f51a249216
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5296918
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Ryan Beltran <ryanbeltran@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed