ccc: Introduce ToolChains for mapping Actions to Tools which can

ccc: Introduce ToolChains for mapping Actions to Tools which can
perform them.

 - A ToolChain is a coherent set of tools use in a compilation
   process. The idea is that a ToolChain holds roughly the information
   (specs, search paths, etc.) that is in a single gcc binary.

 - The default ToolChain is selected by the host and will generally
   correspond to what the default system compiler would do. However,
   this can be over-riden for a variety of purposes, for example the
   by the driver driver or for testing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62021 91177308-0d34-0410-b5e6-96231b3b80d8