[clang-tidy] adding "--config-file=<file-path>" to specify custom config file.

Let clang-tidy to read config from specified file.
Example:
$ clang-tidy --config-file=/some/path/myTidyConfig --list-checks --
...this will read config from '/some/path/myTidyConfig'.

ClangTidyMain.cpp reads ConfigFile into string and then assigned read data to 'Config' i.e. makes like '--config' code flow internally.

May speed-up tidy runtime since now it will just look-up <file-path>
instead of searching ".clang-tidy" in parent-dir(s).

Directly specifying config path helps setting build dependencies.

Thanks to @DmitryPolukhin for valuable suggestion. This patch now propose
change only in ClangTidyMain.cpp.

Reviewed By: DmitryPolukhin

Differential Revision: https://reviews.llvm.org/D89936