Use DiagnosticInfo facility.

Summary:
The sample profiler pass emits several error messages. Instead of
just aborting the compiler with report_fatal_error, we can emit
better messages using DiagnosticInfo.

This adds a new sub-class of DiagnosticInfo to handle the sample
profiler.

Reviewers: chandlerc, qcolombet

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3086

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203976 91177308-0d34-0410-b5e6-96231b3b80d8