[analyzer] NFC: Move resetDiagnosticLocationToMainFile() to BugReporter.

This method of PathDiagnostic is a part of Static Analyzer's particular
path diagnostic construction scheme. As such, it doesn't belong to
the PathDiagnostic class, but to the Analyzer.

Differential Revision: https://reviews.llvm.org/D67418

llvm-svn: 371660