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

[analyzer] Refactor: make PathDiagnosticLocation responsible for validation of SourceLocations (commit 5 of ?):
 - Get rid of PathDiagnosticLocation(SourceRange r,..) constructor by providing a bunch of create methods.
 - The PathDiagnosticLocation(SourceLocation L,..), which is used by crate methods, will eventually become private.
 - Test difference is in the case when the report starts at the beginning of the function. We used to represent that point as a range of the very first token in the first statement. Now, it's just a single location representing the first character of the first statement.

llvm-svn: 139932