Start keeping track of what work was done to detect a GCC installation

on the system, and report it when running the driver in verbose mode.
Without this it is essentially impossible to understand why a particular
GCC toolchain is used by Clang for libstdc++, libgcc, etc.

This also required threading a hook through the toolchain layers for
a specific toolchain implementation to print custom information under
'clang -v'. The naming here isn't spectacular. Suggestions welcome.

llvm-svn: 187427