[clang-tools-extra] [clang-doc] Add helpers for Template config (PR #138062)

[Nico Weber via cfe-commits]

nico wrote:

Reading files from unit tests is a bit of an anti-pattern: It requires putting the absolute path to the tests somewhere (you're doing this with a config file here), and it means you easily run tests on a different machine than the one you're building on. Could you restructure this test to not read a file? Or, if you need to run a file, can you do that from a lit test, where the path from build artifacts and test inputs is abstracted away enough that the build/test separation still works?

https://github.com/llvm/llvm-project/pull/138062