Update branches for new master-next scheme.

Combine the "next" and "next-upstream" checkout schemes since the new
approach for maintaining master-next is to building against the
"upstream-with-swift" Clang/LLVM branches. We do not plan to use the
"stable-next" branches anymore. (We can delete those branches after
this transition has had time to settle out.) I have also merged the
"upstream" scheme with "next-upstream" because building the combination
of branches previously in the scheme is not something we expect to work
now.

Author: bob-wilson

utils/update-checkout-config.json

@@ -53,52 +53,14 @@
             }
         },
         "next" : {
-            "aliases": ["next", "master-next", "stable-next"],
-            "repos": {
-                "llvm": "stable-next",
-                "clang": "stable-next",
-                "compiler-rt": "stable-next",
-                "swift": "master-next",
-                "lldb": "stable-next",
-                "cmark": "master",
-                "llbuild": "master",
-                "swiftpm": "master",
-                "swift-corelibs-xctest": "master",
-                "swift-corelibs-foundation": "master",
-                "swift-corelibs-libdispatch": "master",
-                "swift-integration-tests": "master",
-                "swift-xcode-playground-support": "master",
-                "ninja": "release"
-            }
-        },
-        "next-upstream" : {
-            "aliases": ["next-upstream"],
+            "aliases": ["next", "master-next", "stable-next", "upstream", "next-upstream"],
             "repos": {
                 "llvm": "upstream-with-swift",
                 "clang": "upstream-with-swift",
                 "compiler-rt": "upstream-with-swift",
                 "swift": "master-next",
                 "lldb": "upstream-with-swift",
                 "cmark": "master",
-                "llbuild": "master-next",
-                "swiftpm": "master-next",
-                "swift-corelibs-xctest": "master-next",
-                "swift-corelibs-foundation": "master-next",
-                "swift-corelibs-libdispatch": "master-next",
-                "swift-integration-tests": "master-next",
-                "swift-xcode-playground-support": "master-next",
-                "ninja": "release"
-            }
-        },
-        "upstream": {
-            "aliases": ["upstream"],
-            "repos": {
-                "llvm": "upstream-with-swift",
-                "clang": "upstream-with-swift",
-                "compiler-rt": "upstream-with-swift",
-                "swift": "master",
-                "lldb": "upstream-with-swift",
-                "cmark": "master",
                 "llbuild": "master",
                 "swiftpm": "master",
                 "swift-corelibs-xctest": "master",