Add ability to override JIT expr compiler options.

Runtimes should be able to pass custom compilation options to the JIT for their stack frame. This patch adds a custom expression options member class to LanguageOptions, and modifies the clang expression evaluator to check the current runtime for those options. If those options are available on the runtime, they are passed to the clang compiler.

Committed for Luke Drummond.
Differential Revision: http://reviews.llvm.org/D15527

llvm-svn: 259644