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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187427 91177308-0d34-0410-b5e6-96231b3b80d8