Provide user name in ClangTidyOptions.

Summary:
This adds the ClangTidyOptions::User field and fills it from the USER
or the USERNAME environment variable, if possible. The FileOptionsProvider now
takes "default" options instead of "fallback" options, as it now uses these when
an option is not set in the configuration file (one exception is the checks
list).

Reviewers: bkramer, klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5440

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@218402 91177308-0d34-0410-b5e6-96231b3b80d8