clang driver toolchain refactoring

In this patch, the file static method addProfileRT is
moved to be a virtual member function of base ToolChain class.
This allows derived toolchain to override the default behavior
easily and make it consistent with Darwin toolchain (a TODO was
added for this refactoring - now removed). A new helper method
is also introduced to test if instrumentation profile option
is turned on or not.

Differential Revision: http://reviews.llvm.org/D13326

llvm-svn: 250994