When mangling sugared types for DWARF debug info, we would
occassionally mix generic parameter types from different
generic environments. Since the mangling for a generic
parameter type only recorded the depth and the index, even
for distinct sugared forms, the remangler would produce a
more 'compact' mangling, by folding together generic parameters
that have the same depth/index, but distinct sugarings in the
AST.
Prevent this from happening by desugaring DWARF types the
correct amount, substituting away generic parameters while
preserving everything else.
Also, re-enable the round-trip verification with the remangler.
Fixes <rdar://problem/59496022>, <https://bugs.swift.org/browse/SR-12204>.
