SIL: Track target formal type for casts

SIL type lowering erases DynamicSelfType, so we generate
incorrect code when casting to DynamicSelfType. Fixing this
requires a fair amount of plumbing, but most of the
changes are mechanical.

Note that the textual SIL syntax for casts has changed
slightly; the target type is now a formal type without a '$',
not a SIL type.

Also, the unconditional_checked_cast_value and
checked_cast_value_br instructions now take the _source_
formal type as well, just like the *_addr forms they are
intended to replace.

lib/Serialization/Serialization.cpp

@@ -771,7 +771,6 @@ void Serializer::writeBlockInfoBlock() {
   BLOCK_RECORD(sil_block, SIL_VTABLE);
   BLOCK_RECORD(sil_block, SIL_VTABLE_ENTRY);
   BLOCK_RECORD(sil_block, SIL_GLOBALVAR);
-  BLOCK_RECORD(sil_block, SIL_INST_CAST);
   BLOCK_RECORD(sil_block, SIL_INIT_EXISTENTIAL);
   BLOCK_RECORD(sil_block, SIL_WITNESS_TABLE);
   BLOCK_RECORD(sil_block, SIL_WITNESS_METHOD_ENTRY);