[ELF] - Linkerscript: fixed non-determinism when handling MEMORY.

When findMemoryRegion do search to find a region for output section it
iterates over MemoryRegions which is DenseMap and so does not
guarantee iteration in insertion order. As a result selected region depends
on its name and not on its definition position
Testcase shows the issue, patch fixes it. Behavior after applying the patch
seems consistent with bfd.

Differential revision: https://reviews.llvm.org/D39544

llvm-svn: 317307