[analyzer] Create generic SMT Expr class

Summary:
New base class for all future SMT Exprs.

No major changes except moving `areEquivalent` and `getFloatSemantics` outside of `Z3Expr` to keep the class minimal.

Reviewers: NoQ, george.karpenkov

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D49551

llvm-svn: 337917