Revert "Revert r275027 - Let FuncAttrs infer the 'returned' argument attribute"

Revert "Revert r275027 - Let FuncAttrs infer the 'returned' argument attribute"

This reverts commit r275042; the initial commit triggered self-hosting failures
on ARM/AArch64. James Molloy identified the problematic backend code, which has
been disabled in r275677. Trying again...

Original commit message:

Let FuncAttrs infer the 'returned' argument attribute

A function can have one argument with the 'returned' attribute, indicating that
the associated argument is always the return value of the function. Add
FuncAttrs inference logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275678 91177308-0d34-0410-b5e6-96231b3b80d8