Retry^3 "[ProfileData] (llvm) Use Error in InstrProf and Coverage, NFC"

Transition InstrProf and Coverage over to the stricter Error/Expected
interface.

Changes since the initial commit:
- Fix error message printing in llvm-profdata.
- Check errors in loadTestingFormat() + annotateAllFunctions().
- Defer error handling in InstrProfIterator to InstrProfReader.
- Remove the base ProfError class to work around an MSVC ICE.

Differential Revision: http://reviews.llvm.org/D19901

llvm-svn: 270020