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

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@193875 91177308-0d34-0410-b5e6-96231b3b80d8