Split the DiagnosticInfo class into two disjoint classes:

Split the DiagnosticInfo class into two disjoint classes:
one for building up the diagnostic that is in flight (DiagnosticBuilder)
and one for pulling structured information out of the diagnostic when
formatting and presenting it.

There is no functionality change with this patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59849 91177308-0d34-0410-b5e6-96231b3b80d8