Add METADATA files for new filtered subtrees. (#457) (#458)

Add METADATA files for new filtered llvm-project subtrees; compiler-rt,
libcxx, libcxxabi, and libunwind which replace the llvm-project subtree.

b/284200202

(cherry picked from commit 6902aef)

third_party/llvm-project/compiler-rt/METADATA

@@ -0,0 +1,21 @@
+name: "compiler-rt"
+description:
+  "Filtered subtree at third_party/llvm-project/compiler-rt."
+
+third_party {
+  url {
+    type: LOCAL_SOURCE
+    value: "https://cobalt.googlesource.com/third_party/llvm-project/compiler-rt_filtered_mirror"
+  }
+  url {
+    type: GIT
+    value: "https://github.com/llvm/llvm-project.git"
+  }
+  # Closest commit hash to llvm-project's 39da55e8f548a11f7dadefa73ea73d809a5f1729.
+  version: "1066968d931196d4ab5e1dfb54d4fd53f3cc024f"
+  last_upgrade_date {
+    year: 2023
+    month: 1
+    day: 17
+  }
+}

third_party/llvm-project/libcxx/METADATA

@@ -0,0 +1,21 @@
+name: "libcxx"
+description:
+  "Filtered subtree at third_party/llvm-project/libcxx."
+
+third_party {
+  url {
+    type: LOCAL_SOURCE
+    value: "https://cobalt.googlesource.com/third_party/llvm-project/libcxx_filtered_mirror"
+  }
+  url {
+    type: GIT
+    value: "https://github.com/llvm/llvm-project.git"
+  }
+  # Closest commit hash to llvm-project's 39da55e8f548a11f7dadefa73ea73d809a5f1729.
+  version: "5c30f21b42b422fe507d923ce32f9f268f10d88b"
+  last_upgrade_date {
+    year: 2023
+    month: 1
+    day: 17
+  }
+}

third_party/llvm-project/libcxxabi/METADATA

@@ -0,0 +1,21 @@
+name: "libcxxabi"
+description:
+  "Filtered subtree at third_party/llvm-project/libcxxabi."
+
+third_party {
+  url {
+    type: LOCAL_SOURCE
+    value: "https://cobalt.googlesource.com/third_party/llvm-project/libcxxabi_filtered_mirror"
+  }
+  url {
+    type: GIT
+    value: "https://github.com/llvm/llvm-project.git"
+  }
+  # Closest commit hash to llvm-project's 39da55e8f548a11f7dadefa73ea73d809a5f1729.
+  version: "ae56bdc07c05dc3920476eef7adad00fd2548782"
+  last_upgrade_date {
+    year: 2023
+    month: 1
+    day: 17
+  }
+}

third_party/llvm-project/libunwind/METADATA

@@ -0,0 +1,21 @@
+name: "libunwind"
+description:
+  "Filtered subtree at third_party/llvm-project/libunwind."
+
+third_party {
+  url {
+    type: LOCAL_SOURCE
+    value: "https://cobalt.googlesource.com/third_party/llvm-project/libunwind_filtered_mirror"
+  }
+  url {
+    type: GIT
+    value: "https://github.com/llvm/llvm-project.git"
+  }
+  # Closest commit hash to llvm-project's 39da55e8f548a11f7dadefa73ea73d809a5f1729.
+  version: "4cc5f231b5b0174cdd5b0f7fc3be5a595be0009d"
+  last_upgrade_date {
+    year: 2023
+    month: 1
+    day: 17
+  }
+}