[CAS|LLDB] Cleanup diagnostics in ExplicitCASModuleLoader
While configuring LLDB to use ExplicitCASModuleLoader, it enounters some
error diagnostics that causes module loading to fail, rather than fall
back to the next module loader.
This commit clears:
* Allow ExplicitCASModuleLoader to be initialized with an empty explicit
module map CASID
* Do not trigger error diagnostics when module lookup failed in module
dependency scanner.
rdar://166490023
