[opaque pointer types] Add an explicit pointee type to alias records in the IR

[opaque pointer types] Add an explicit pointee type to alias records in the IR

Since aliases actually use and verify their explicit type already, no
further invalid testing is required here. The
invalid.test:ALIAS-TYPE-MISMATCH case catches errors due to emitting a
non-pointee type in the new format or a non-pointer type in the old
format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247952 91177308-0d34-0410-b5e6-96231b3b80d8