Use plain Apache 2.0 license for mirrors. (#7407)

Our mirror repositories contain vim scripts and textmate grammars,
neither of which have any need for the LLVM exception, so we can use the
base Apache 2.0 license there with no loss of relevant permissions. This
is a compatible license, so providing these repositories under these
license terms is valid.

Other than removing the LLVM exception, the new license file has one
other change compared to LICENSE: the amount of whitespace on some lines
is reduced from four spaces to three. This makes the new license
*exactly* match the Apache 2.0 license, byte-for-byte.

This is important because the license checking done by GitHub's linguist
project doesn't recognize Apache-2.0-with-LLVM-exception as an
acceptable license, but does allow plain Apache-2.0.

.pre-commit-config.yaml

@@ -232,6 +232,7 @@ repos:
               .bazelversion|
               .github/pull_request_template.md|
               .python-version|
+              LICENSE.*|
               compile_flags.txt|
               github_tools/requirements.txt|
               third_party/.*|

scripts/sync_repos.sh

@@ -44,10 +44,13 @@ for dir in "${!MIRRORS[@]}"; do
   git rm --ignore-unmatch -r .
   git status
 
+  # Restore the license file. The mirrors use the Apache 2.0 license, which is
+  # compatible with our license but excludes the LLVM exception.
+  git checkout HEAD LICENSE
+
   # Copy the basic framework from the origin repository.
   cp "$ORIGIN_DIR/.gitignore" \
     "$ORIGIN_DIR/CODE_OF_CONDUCT.md" \
-    "$ORIGIN_DIR/LICENSE" \
     .
 
   # Copy the mirrored directory. We use `rsync` to get a more reliable way of