[clang-tidy] add option to specify build path

Summary:
compile_commands.json is usually generated in the build directory.
Projects like LLVM/Clang enforce out-of-source builds.
This option allow allow such projects to work out of the box, without
moving the compilation database manually.

The naming of the option is similar to the one use by other tools:

    clang-{check,modernize,query,rename,tidy} -p=<build_path> <...>

Reviewers: alexfh

Differential Revision: http://reviews.llvm.org/D13199

llvm-svn: 248723