[-Wunsafe-buffer-usage] Move the whole analysis to the end of a translation unit

The unsafe-buffer analysis requires a complete view of the translation
unit (TU) to be conservative. So the analysis is moved to the end of a
TU.

A summary of changes made: add a new `IssueWarnings` function in
`AnalysisBasedWarnings.cpp` for TU-based analyses. So far
[-Wunsafe-buffer-usage] is the only analysis using it but there could
be more.  `Sema` will call the new `IssueWarnings` function at the end
of parsing a TU.

Reviewed by: NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D146342