[analyzer] Don't crash if the analyzer-constraint is set to Z3, but llvm is not built with it

Exactly what it says on the tin! We had a nasty crash with the following incovation:

$ clang --analyze -Xclang -analyzer-constraints=z3 test.c
fatal error: error in backend: LLVM was not compiled with Z3 support, rebuild with -DLLVM_ENABLE_Z3_SOLVER=ON
... <stack trace> ...

Differential Revision: https://reviews.llvm.org/D120325