Bump useable clang version to 318

Build 211 can build most things, but I've seen intermittent
miscompilation and slower code. Also it comes from Xcode 4.2, so most
people don't have it anyway. Let's use 218 as the minimum viable
version.
CompilerSelector: don't use non-existent compilers
Isolate compiler selector tests
Add new tests for fails_with DSL