SanitizerArgs: add ability to filter/diagnose unsupported sanitizers.

The thread, memory, dataflow and function sanitizers are now diagnosed if
enabled explicitly on an unsupported platform.  Unsupported sanitizers which
are enabled implicitly (as part of a larger group) are silently disabled.  As a
side effect, this makes SanitizerArgs parsing toolchain-dependent (and thus
essentially reverts r188058), and moves SanitizerArgs ownership to ToolChain.

Differential Revision: http://llvm-reviews.chandlerc.com/D1990