clang-check: Enable specification of additional compiler arguments

Summary:
This adds two command-line parameters: -extra-arg and -extra-arg-before, which
enable the user to pass additional parameters to the compiler command.

Reviewers: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D919

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183320 91177308-0d34-0410-b5e6-96231b3b80d8