Prune #includes from llvm/Linker.h and llvm/System/Path.h,

Prune #includes from llvm/Linker.h and llvm/System/Path.h,
forcing them down into various .cpp files.

This change also:
1. Renames TimeValue::toString() and Path::toString() to ::str()
   for similarity with the STL.
2. Removes all stream insertion support for sys::Path, forcing
   clients to call .str().
3. Removes a use of Config/alloca.h from bugpoint, using smallvector
   instead.
4. Weans llvm-db off <iostream>

sys::Path really needs to be gutted, but I don't have the desire to
do it at this point.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79869 91177308-0d34-0410-b5e6-96231b3b80d8