swift234

[Mangler] Add methods for encoding already-mangled symbols.

This commit is related to the work of encoding mangled names more efficiently by
compressing them. This commit adds two new methods to the mangler that allows it
to identify requests to mangle strings that are already mangled.  Right now the
mangler does not do anything with this information.

This API is needed in all of the places in the compiler where we compose mangled
names. For example, when the optimizer is cloning functions it adds a prefix to
an existing name.

I verified that this change is correct by adding a compress/decompress methods
that add a prefix to the mangled names with assertions to catch compression of
already compressed symbols or decompression of non-compressed named.  I plan to
commit the verification code together with the compression implementation later
on.