Remove the Tools DensMap from the toolchain.

Each toolchain has a set of tools, but they are all of known types. It can
have a linker, an assembler, a "clang" (compile, analyze, ...) a non-clang
compiler, etc.

Instead of keeping a map, just have member variable for each type of tool.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177479 91177308-0d34-0410-b5e6-96231b3b80d8