[MCJIT] Improve documentation and error handling for MCJIT::runFunction.

ExecutionEngine::runFunction is supposed to allow execution of arbitrary
function types, but MCJIT can only reasonably support a limited subset of
main-linke function types. This patch documents this limitation, and fixes
MCJIT::runFunction to abort with a meaningful error at runtime if called with
an unsupported function type.

llvm-svn: 278348