PR19598: Provide the ability to RAUW a declaration with itself, creating a non-temporary copy and using that to RAUW.

Also, provide the ability to create temporary and non-temporary
declarations, as not all declarations may be replaced by definitions
later on.

This provides the necessary infrastructure for Clang to fix PR19598,
leaking temporary MDNodes in Clang's debug info generation.
PR19598: Ensure temporary metadata nodes used in debug info are destroyed.

CGDebugInfo and DIBuilder were lax in their handling of temporary
MDNodes. All temporary nodes need to be deleted, which means they need
to be RAUW'd with a permanent node. This was not happening.

To ensure this, leverage DIBuilder's new ability to create both
permanent and temporary declarations. Ensure all temporary declarations
are RAUW'd, even with itself. (DIDescriptor::RAUW handles the case where
it is replaced with itself and creates a new, duplicate permanent node
to replace itself with)

This means that all temporary declarations must be added to the
ReplacementMap even if they're never upgraded to definitions - so move
the point of insertion into the map to the point of creation of the
declarations.