swift234

Change SILDeserializer to use StringRef instead of Identifier as its internal key.

This enables us to lookup a function from a key by avoiding the need to create
an identifier inside the OnDiskHashTable structure. Doing so would require an
ASTContext, something that is not available therein.

As a side effect this also makes OnDiskHashTable more efficient by just using a
StringRef reference instead of creating a uniqued identifier in the AST for
every deserialized node in the table.

Swift SVN r14169