Allow a ToolChain to compute the path of a compiler-rt's component.

Summary:
This patch moves getCompilerRT() from the clang::driver::tools namespace to
the ToolChain class. This is needed for multilib toolchains that need to
place their libraries in Clang's resource directory with a layout that is
different from the default one.

Reviewers: atanasyan, rsmith

Subscribers: tberghammer, danalbert, srhines, cfe-commits

Differential Revision: http://reviews.llvm.org/D13339

llvm-svn: 249030