[C-API][ORC] Add LLVMOrcExecutionSessionLookup -- generic async symbol lookup.

An API to wrap ExecutionSession::lookup, this allows C API clients to use async
lookup.

The immediate motivation for adding this is to simplify upcoming
definition-generator unit tests.

As we're adding more tests that need to convert between C and C++ flag values
this commit adds helper functions to support this. This patch also updates the
CAPIDefinitionGenerator to use these new utilities.