[Bazel] Change external_zlib attribute to string

When using `llvm_zlib_external` rule with `external_zlib` attribute set to a
label referring to the main repository, like `@//third_party/zlib`, it will be
replaced with `//third_party/zlib` after template substitution. This will then
attempt to find `//third_party/zlib` within the local repository
`@llvm_zlib//third_party/zlib`, which does not exist, rather than the intended
reference back to the main repository. The issue appears to be that the
conversion of `Label` type to string with
`str(repository_ctx.attr.external_zlib)`, which is causing the main repository
qualifier to be lost.

This diff fixes the issue by changing the `external_zlib` attribute to
`attr.string` type rather than `attr.label`.

In future a more elegant solution may be possible that preserves use of the
`Label` type, depending on resolution of the issue
https://github.com/bazelbuild/bazel/issues/13731.

Ported from Github PR https://github.com/google/llvm-bazel/pull/236.

Reviewed By: GMNGeoffrey

Differential Revision: https://reviews.llvm.org/D106606

utils/bazel/zlib.bzl

@@ -24,7 +24,7 @@ def _llvm_zlib_external_impl(repository_ctx):
         "BUILD",
         repository_ctx.attr._external_build_template,
         substitutions = {
-            "@external_zlib_repo//:zlib_rule": str(repository_ctx.attr.external_zlib),
+            "@external_zlib_repo//:zlib_rule": repository_ctx.attr.external_zlib,
         },
         executable = False,
     )
@@ -36,7 +36,7 @@ llvm_zlib_external = repository_rule(
             default = Label("//deps_impl:zlib_external.BUILD"),
             allow_single_file = True,
         ),
-        "external_zlib": attr.label(
+        "external_zlib": attr.string(
             doc = "The dependency that should be used for the external zlib library.",
             mandatory = True,
         ),