Remove llvm::MemoryBuffer const_casts

Summary:
llvm has grown a WritableMemoryBuffer class, which is convertible
(inherits from) a MemoryBuffer. We can use it to avoid conts_casting the
buffer contents when we want to write to it.

Reviewers: dblaikie, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41387

llvm-svn: 321167