Cache the value for absolute path in FileSpec.

Checking if a path is absolute can be expensive and currently the result is not cached in the FileSpec object. This patch adds caching and also code to clear the cache if the file is modified.

Differential Revision: https://reviews.llvm.org/D130396

GitOrigin-RevId: dc0f452e85e572f1e824c1eb3cdaa060d326f0c3
3 files changed