[ASTUnit] Don't let the preamble diagnostics out-live the CompilerInstance that created them,
this is inherently unsafe.

Instead get the diagnostic info into a SourceManager-independent form.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202471 91177308-0d34-0410-b5e6-96231b3b80d8