[clang-tidy] Move -extra-arg handling to CommonOptionsProvider

Summary:
Handle -extra-arg and -extra-arg-before options in the
CommonOptionsProvider so they can be used in all clang tools. Adjust arguments
in a CompilationDatabase wrapper instead of adding ArgumentsAdjuster to the
tool.

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6073

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221248 91177308-0d34-0410-b5e6-96231b3b80d8
Driver: Pass some std::strings by reference instead of value

No functional change intended. This fixes PR21463.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221249 91177308-0d34-0410-b5e6-96231b3b80d8