[clang] [clang-tools-extra] In compilation databases, add support for relative directories (PR #69856)

[via cfe-commits]

Overhatted wrote:

@MaskRay Can you take a look at this PR? For now I'm not looking for a full review, just an answer to my question of whether supporting a relative directory in the "directory" field of a compilation database is an acceptable change. Thanks. 

https://github.com/llvm/llvm-project/pull/69856