IR: Remove MDTupleTypedArrayWrapper::operator MDTuple*()

Remove `MDTupleTypedArrayWrapper::operator MDTuple*()`, since it causes
ambiguity (at least in some [1] compilers [2]) when using indexes to
`MDTupleTypedArrayWrapper::operator[](unsigned)` that are convertible to
(but not the same as) `unsigned`.

[1]: http://lab.llvm.org:8011/builders/sanitizer-windows/builds/2308
[2]: http://lab.llvm.org:8011/builders/clang-cmake-mips/builds/4442

llvm-svn: 234326