Revert r302469 "Make it illegal for two Functions to point to the same DISubprogram"

Revert r302469 "Make it illegal for two Functions to point to the same DISubprogram"

This caused PR32977.

Original commit message:

> Make it illegal for two Functions to point to the same DISubprogram
>
> As recently discussed on llvm-dev [1], this patch makes it illegal for
> two Functions to point to the same DISubprogram and updates
> FunctionCloner to also clone the debug info of a function to conform
> to the new requirement. To simplify the implementation it also factors
> out the creation of inlineAt locations from the Inliner into a
> general-purpose utility in DILocation.
>
> [1] http://lists.llvm.org/pipermail/llvm-dev/2017-May/112661.html
> <rdar://problem/31926379>
>
> Differential Revision: https://reviews.llvm.org/D32975

llvm-svn: 302533