Persist the TextDiagnostic object across multiple diagnostics as long as

Persist the TextDiagnostic object across multiple diagnostics as long as
the SourceManager doesn't change, and the source files don't change.
This greatly simplifies the interfaces and interactions. The lifetime of
the TextDiagnostic object forms the 'session' over which we attempt to
condense and deduplicate information in diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142104 91177308-0d34-0410-b5e6-96231b3b80d8