Replace `element_type*` handles in HLSLExternalSemaSource with
`__hlsl_resource_t` builtin type.
The handle used to be defined as `element_type*` which was used by the
provisional subscript operator implementation. Now that the handle is
`__hlsl_resource_t` the subscript placeholder implementation was updated
to add `element_type* e;` field to the resource struct. and return a
reference to that. This field is just a temporary workaround until the
indexing is implemented properly in llvm/llvm-project#95956, at which
point the field will be removed. This seemed like a better solution than
disabling many of the existing tests that already use the `[]` operator.
One test has to be disabled nevertheless because an error based on
interactions of const and template instantiation (potential bug that can
be investigated once indexing is implemented the right way).
Fixes#84824
e20bf28987