[analyzer] Add option to set maximum symbol complexity threshold

Summary:
This adds an option, max-symbol-complexity, so an user can set the maximum symbol complexity threshold.

Note that the current behaviour is equivalent to max complexity = 0, when taint analysis is not enabled and tests show that in a number of tests, having complexity = 25 yields the same results as complexity = 10000.

This patch was extracted and modified from Dominic Chen's patch, D35450.

Reviewers: george.karpenkov, NoQ, ddcc

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D49093

llvm-svn: 336671