llvm-profdata: Reduce memory usage by using Error callback rather than member

Reduces llvm-profdata memory usage on a large profile from 7.8GB to 5.1GB.

The ProfData API now supports reporting all the errors/warnings rather
than only the first, though llvm-profdata ignores everything after the
first for now to preserve existing behavior. (if there's a desire for
other behavior, happy to implement that - but might be as well left for
a separate patch)

Reviewers: davidxl

Differential Revision: https://reviews.llvm.org/D35149

llvm-svn: 307516