Use InitLLVM in clang-tidy

Update clang-tidy to use InitLLVM, like several other llvm tools that
were previously updated. On Windows, this allows clang-tidy to operate
on arguments containing characters which cannot be represented in the
system's ANSI code page such as filenames with Unicode characters.

Fixes bugzilla bug 43751.

Patch by Tristan Labelle.

Differential Revision: https://reviews.llvm.org/D70694