[Support] Add WritableMemoryBuffer::getNewMemBuffer

Summary:
The idea is that it would replace
(non-Writable)MemoryBuffer::getNewMemBuffer, which is quite useless
unless you const_cast its contents to write to it (which all (both)
callers of this function were doing). This patch also fixes one of the usages in
COFFWriter. After fixing the other usage in clang, I plan to delete the old
function.

Reviewers: dblaikie, Bigcheese

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41540

llvm-svn: 322094