[analyzer] Memoize complexity of SymExpr

Summary:
This patch introduces a new member to SymExpr, which stores the symbol complexity, avoiding recalculating it every time computeComplexity() is called.

Also, increase the complexity of conjured Symbols by one, so it's clear that it has a greater complexity than its underlying symbols.

Reviewers: NoQ, george.karpenkov

Reviewed By: NoQ, george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D49232

llvm-svn: 337472