Fix a major oversight in the comparison of standard conversion

sequences, where we would occasionally determine (incorrectly) that
one standard conversion sequence was a proper subset of another when,
in fact, they contained completely incomparable conversions. 

This change records the types in each step within a standard
conversion sequence, so that we can check the specific comparison
types to determine when one sequence is a proper subset of the
other. Fixes this testcase (thanks, Anders!), which was distilled from
PR6095 (also thanks to Anders).

llvm-svn: 94660