Re-apply "InstrProf: Add unit tests for the profile reader and writer"

Have the InstrProfWriter return a MemoryBuffer instead of a
std::string. This fixes the alignment issues the reader would hit, and
it's a more appropriate type for this anyway.

I've also removed an ugly helper function that's not needed since
we're allowing initializer lists now, and updated some error code
checks based on MSVC's issues with r229473.

This reverts r229483, reapplying r229478.

llvm-svn: 229602