Swift: fix cmake generation

The bazel -> cmake generator is currently not capable of handling
separate included generated cmake files making use of common C/C++
dependencies.

To work around this limitation, a single generated cmake is now in
place. Long-term, we should either:
* make the cmake generator handle common dependencies gracefully, or
* make the cmake generation aspect travel up `pkg_` rules `srcs`
  attributes
so to avoid having to list the targets to be generated in the top-level
`BUILD` file.

Other things fixed:
* removed some warning spam about redefined `BAZEL_CURRENT_REPOSITORY`
* fixed the final link step, that was failing because `libswiftCore.so`
  was not being linked.

swift/logging/tests/assertion-diagnostics/BUILD.bazel

@@ -4,7 +4,7 @@ load("//misc/bazel/cmake:cmake.bzl", "generate_cmake")
 swift_cc_binary(
     name = "assert-false",
     srcs = ["AssertFalse.cpp"],
-    visibility = ["//visibility:private"],
+    visibility = ["//swift:__pkg__"],
     deps = [
         "//swift/logging",
     ],
@@ -14,12 +14,9 @@ py_test(
     name = "test",
     size = "small",
     srcs = ["test.py"],
+    data = [
+        "diagnostics.expected",
+        ":assert-false",
+    ],
     deps = ["//swift/integration-tests:integration_tests"],
-    data = [":assert-false", "diagnostics.expected"],
-)
-
-generate_cmake(
-    name = "cmake",
-    targets = [":assert-false"],
-    visibility = ["//visibility:public"],
 )