[analyzer] Refactor: make PathDiagnosticLocation responsible for validation of...

[analyzer] Refactor: make PathDiagnosticLocation responsible for validation of SourceLocations (commit 2 of ?):
- Fix a fixme and move the logic of creating a PathDiagnosticLocation corresponding to a ProgramPoint into a PathDiagnosticLocation constructor.
- Rename PathDiagnosticLocation::create to differentiate from the added constructor.

llvm-svn: 139825