[Support] Add WritableMemoryBuffer class

Summary:
The motivation here is LLDB, where we need to fixup relocations in
mmapped files before their contents can be read correctly.  The
MemoryBuffer class does exactly what we need, *except* that it maps the
file in read-only mode.

WritableMemoryBuffer reuses the existing machinery for opening and
mmapping a file. The only difference is in the argument to the
mapped_file_region constructor -- we create a private copy-on-write
mapping, so that we can make changes to the mapped data, but the changes
aren't carried over to the underlying file.

This patch is based on an initial version by Zachary Turner.

Reviewers: mehdi_amini, rnk, rafael, dblaikie, zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40291

llvm-svn: 321071