[analyzer] Improved cmake configuration for Z3

Summary:
Enhanced support for Z3 in the cmake configuration of clang; now it is possible to specify any arbitrary Z3 install prefix (CLANG_ANALYZER_Z3_PREFIX) to cmake with lib (or bin) and include folders. Before the patch only in cmake default locations
were searched (https://cmake.org/cmake/help/v3.4/command/find_path.html).

Specifying any CLANG_ANALYZER_Z3_PREFIX will force also CLANG_ANALYZER_BUILD_Z3 to ON.

Removed also Z3 4.5 version requirement since it was not checked, and now Clang works with Z3 4.7

Reviewers: NoQ, george.karpenkov, mikhail.ramalho

Reviewed By: george.karpenkov

Subscribers: rnkovacs, NoQ, esteffin, george.karpenkov, delcypher, ddcc, mgorny, xazax.hun, szepet, a.sidorin, Szelethus

Tags: #clang

Differential Revision: https://reviews.llvm.org/D50818

llvm-svn: 344464