[MemoryBuffer] Allow optionally specifying desired buffer alignment

Underlying data may have requirements/expectations/etc. about
the run-time alignment. WritableMemoryBuffer currently uses
a 16 byte alignment, which works for many situations but not all.
Allowing a desired alignment makes it easier to reuse WritableMemoryBuffer
in situations of special alignment, and also removes a problem when
opening files with special alignment constraints. Large files generally
get mmaped, which has ~page alignment, but small files go through
WritableMemoryBuffer which has the much smaller alignment guarantee.

Differential Revision: https://reviews.llvm.org/D137820