[Bazel] Use `LLVM_VERSION` from `llvm/CMakeLists.txt`

* Generate `//:vars.bzl` from `llvm/CMakeLists.txt`

`_extract_cmake_settings()` generates `//:vars.bzl` in `llvm_configure()`.
It would be easier to use external commands like sed(1) and python.
For portability, I think the parser should run on Starlark.

`@llvm-project//:vars.bzl` may be loaded from both WORKSPACE and BUILD.
At the moment, `vars.bzl` provides some values as string.

- CMAKE_CXX_STANDARD = "17"
- LLVM_VERSION_MAJOR = "16"
- LLVM_VERSION_MINOR = "0"
- LLVM_VERSION_PATCH = "0"
- LLVM_VERSION = "16.0.0"
- llvm_vars = (dict of these values)

`CMAKE_CXX_STANDARD` may be used to configure toolchain.

* Use `//vars.bzl` for each BUILD files

It would be smarter if the BUILD phase could generate `llvm-config.h`.
Since I am afraid of the discussion in D126581, I just remove
LLVM_VERSION stuff out of the static `llvm-config.h`.

* Eliminate Bazel stuff in 'bump-version.py'

Current version of `bump-version.py` tries to substitute CLANG_VERSION.
It is the reason why I modify bump-version in this change rather than
incoming patch.

Differential Revision: https://reviews.llvm.org/D136392

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