[LoopAccesses] Make raw_string_ostream local in VectorizationReport

Since VectorizationReport will be part of the result of the analysis it
will be stored in a container.  However, one of its members is a
raw_string_ostream which cannot be copy-constructed.

This makes the raw_string_ostream local to the << operator.

This is part of the patchset that converts LoopAccessAnalysis into an
actual analysis pass.

llvm-svn: 229620