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

[analyzer] Refactor: make PathDiagnosticLocation responsible for validation of SourceLocations (commit 2 of ?):
- Modify all PathDiagnosticLocation constructors that take Stmt to also requre LocationContext.
- Add a constructor which should be used in case there is no valid statement/location (it will grab the location of the enclosing function).

llvm-svn: 139763