[MemoryLocation] Move DSE's logic to new MemLoc::getForDest helper (NFC).

DSE has some extra logic to determine the write location of library
calls like str*cpy and str*cat. This patch moves the logic to a new
MemoryLocation:getForDest variant, which takes a call and TLI.

This patch should be NFC, because no other places take advantage of the
new helper yet.

Suggested by @reames post-commit 7eec832d.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D114872