[Layering] Sink Linker.h into a Linker subdirectory to make it
consistent with every other sub-library header in LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203065 91177308-0d34-0410-b5e6-96231b3b80d8
[cleanup] Re-sort the standard library include lines.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203066 91177308-0d34-0410-b5e6-96231b3b80d8