[clangd] Make lit tests work with the internal shell
This makes all of the clangd tests work with the internal shell.
Modifications needed for each test are as follows:
1. system-include-extractor.test was using variable expansion which is
not supported in the internal shell. This patch rewrites it to use
the readfile mechanism along with python. This isn't super pretty but
is readily understandable and there are only two tests across the
monorepo that use this construction, so making it prettier is hard to
justify.
2. include-cleaner-batch-fix.test - Was using $'' construction to create
new lines in a string. Simply replace it with multiple echo commands
to be canonical with the rest of the repository.
3. index-tools.test - Just add IndexBenchmark to the clangd test
depends, so the test now just works unconditionally. This should
significantly increase test coverage at little cost.
Reviewers: ilovepi, HighCommander4, petrhosek, kadircet
Reviewed By: ilovepi
Pull Request: https://github.com/llvm/llvm-project/pull/169539
