When a metadata pack is heap allocated, the pointer's LSB is set.
But lldb is expecting a real pointer. So mask it off.
Companion PR has a test: https://github.com/apple/llvm-project/pull/6961
rdar://110195273
This entry point doesn't exist on older runtimes. Sema enforces
that constrained existentials don't appear in substitution maps,
however IRGen would sometimes try to instantiate metadata even
if the user didn't do that.
Fixes#64657.
Previously it was testing for opened existentials specifically.
We should really teach outlining to handle local archetypes properly.
We'd have to build a generic signature for the lowered type, and that
probably means also adding requirements that are relevant to value
operations, but it would mean outlining would benefit all types, and
it would let us avoid bundling in unnecessary information from the
enclosing generic environment.
A minor side-effect of this is that we no longer bind names to
opened element type values. The names were things like \tau_1_0,
which is not very useful, especially in LLVM IR where \tau is
printed with two UTF-8 escapes.
