Use force checkout so that local files do not cause failure.

[browneee]

Summary

Example of problem:

+ git checkout 53753531bc19bae816cf0cbf9e713ce0333ba091
error: Your local changes to the following files would be overwritten by checkout:
  libcxx/test/std/algorithms/robust_against_adl.compile.pass.cpp
Please commit your changes or stash them before you switch branches.
Aborting

This script then will retry at a greater depth for this problem.

Note that checkout -f with a non-existing revision will exit non-zero.

[browneee]

Add -f on the second checkout command.

[browneee]

Going to push without review to repair this buildbot ASAP: https://lab.llvm.org/buildbot/#/builders/168

zorg/buildbot/builders/sanitizers/buildbot_functions.sh

(
       )
       cd llvm-project
       git fetch --depth $DEPTH origin main
       git clean -fd
       local REV=${BUILDBOT_REVISION}
       if [[  "$REV" != "origin/main" ]] ; then
         # "git fetch --depth 1 origin $REV" does not work with 2.11 on bots
         while true ; do
-          git checkout $REV && break
+          git checkout -f $REV && break
           git rev-list --pretty --max-count=1 origin/main
           git rev-list --pretty --max-parents=0 origin/main
           echo "DEPTH=$DEPTH is too small"
           [[ "$DEPTH" -le "1000000" ]] || exit 1
           DEPTH=$(( $DEPTH * 10 ))
           git fetch --depth $DEPTH origin
         done
       fi
-      git checkout $REV
+      git checkout -f $REV
       git status
       git rev-list --pretty --max-count=1 HEAD
     ) || { echo @@@STEP_EXCEPTION@@@ ; exit 1 ; }
     LLVM=$ROOT/llvm-project/llvm
   fi
 }
 
 function common_stage1_variables {