[analyzer] NFC: Move IssueHash to libAnalysis.

Authored by dergachev.a on Jul 14 2020, 8:26 PM.


[analyzer] NFC: Move IssueHash to libAnalysis.

IssueHash is an attempt to introduce stable warning identifiers
that won't change when code around them gets moved around.
Path diagnostic consumers print issue hashes for the emitted diagnostics.

This move will allow us to ultimately move path diagnostic consumers
to libAnalysis.

Differential Revision: https://reviews.llvm.org/D67421