Renamed bazel-toolchain to toolchains_llvm

[ArchanaShinde1]

Checklist before submitting

• Did you read the contributor guide?
• Did you update any affected documentation?
• Did you write any tests to validate this change?

Description

As part of the transfer to the bazel-contrib org, the repo has been renamed to
toolchains_llvm. This has affected the dynamically generated source archives
which now have a different tree prefix, and different shasum.

Fixes # (issue).

Review process to land

1. All tests and other checks must succeed.
2. At least one maintainer must review and approve.
3. If any maintainer requests changes, they must be addressed.

[cdeepali]

Please update build number.