[clang-tidy] Optional inheritance of file configs from parent directories 

Summary:
Without this patch clang-tidy stops finding file configs on the nearest
.clang-tidy file. In some cases it is not very convenient because it
results in common parts duplication into every child .clang-tidy file.
This diff adds optional config inheritance from the parent directories
config files.

Test Plan:

Added test cases in existing config test.

Reviewers: alexfh, gribozavr2, klimek, hokein

Subscribers: njames93, arphaman, xazax.hun, aheejin, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D75184