[Support] Add WriteThroughMemoryBuffer.

This is like MemoryBuffer (read-only) and WritableMemoryBuffer
(writable private), but where the underlying file can be modified
after writing.  This is useful when you want to open a file, make
some targeted edits, and then write it back out.

Differential Revision: https://reviews.llvm.org/D44230

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327057 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed