Update the LLVM version in use. (#2895)

The prior terminfo and zlib calls are obsolete. I'm adding the zstd library myself as a quick fix, although I want to investigate if we can make better use of [LLVM's workspace](https://github.com/llvm/llvm-project/blob/main/utils/bazel/WORKSPACE) (where the zstd dep comes from).

```
ERROR: .../external/llvm-project/llvm/BUILD.bazel:184:11: no such package '@llvm_zstd//': The repository '@llvm_zstd' could not be resolved: Repository '@llvm_zstd' is not defined and referenced by '@llvm-project//llvm:Support'
```

The particular commit in use fixes a macos build error. https://github.com/llvm/llvm-project/commit/c5f6a287499a816cba5585708999e2c8b134290f

WORKSPACE

@@ -5,6 +5,7 @@
 workspace(name = "carbon")
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
 
 skylib_version = "1.3.0"
 
@@ -112,7 +113,7 @@ http_archive(
 
 # We pin to specific upstream commits and try to track top-of-tree reasonably
 # closely rather than pinning to a specific release.
-llvm_version = "219ba2fb7b0ae89101f3c81a47fe4fc4aa80dea4"
+llvm_version = "c5f6a287499a816cba5585708999e2c8b134290f"
 
 http_archive(
     name = "llvm-raw",
@@ -123,7 +124,7 @@ http_archive(
         "@carbon//bazel/patches/llvm:0002_Added_Bazel_build_for_compiler_rt_fuzzer.patch",
         "@carbon//bazel/patches/llvm:0003_Modernize_py_binary_rule_for_lit.patch",
     ],
-    sha256 = "8b2fa8ae3e434577b4fdd1e91b8990b0651776bd78cf4fbf9b709dcdcdbfbd21",
+    sha256 = "03a8eb4b243846ee037d700b048ec48a87eeef480cb129ab56aa7e0537172b98",
     strip_prefix = "llvm-project-{0}".format(llvm_version),
     urls = ["https://github.com/llvm/llvm-project/archive/{0}.tar.gz".format(llvm_version)],
 )
@@ -132,22 +133,35 @@ load("@llvm-raw//utils/bazel:configure.bzl", "llvm_configure")
 
 llvm_configure(
     name = "llvm-project",
-    repo_mapping = {"@llvm_zlib": "@zlib"},
     targets = [
         "AArch64",
         "X86",
     ],
 )
 
-load("@llvm-raw//utils/bazel:terminfo.bzl", "llvm_terminfo_system")
-
-# We require successful detection and use of a system terminfo library.
-llvm_terminfo_system(name = "llvm_terminfo")
-
-load("@llvm-raw//utils/bazel:zlib.bzl", "llvm_zlib_system")
+# Dependencies copied from
+# https://github.com/llvm/llvm-project/blob/main/utils/bazel/WORKSPACE.
+maybe(
+    http_archive,
+    name = "llvm_zlib",
+    build_file = "@llvm-raw//utils/bazel/third_party_build:zlib-ng.BUILD",
+    sha256 = "e36bb346c00472a1f9ff2a0a4643e590a254be6379da7cddd9daeb9a7f296731",
+    strip_prefix = "zlib-ng-2.0.7",
+    urls = [
+        "https://github.com/zlib-ng/zlib-ng/archive/refs/tags/2.0.7.zip",
+    ],
+)
 
-# We require successful detection and use of a system zlib library.
-llvm_zlib_system(name = "zlib")
+maybe(
+    http_archive,
+    name = "llvm_zstd",
+    build_file = "@llvm-raw//utils/bazel/third_party_build:zstd.BUILD",
+    sha256 = "7c42d56fac126929a6a85dbc73ff1db2411d04f104fae9bdea51305663a83fd0",
+    strip_prefix = "zstd-1.5.2",
+    urls = [
+        "https://github.com/facebook/zstd/releases/download/v1.5.2/zstd-1.5.2.tar.gz",
+    ],
+)
 
 ###############################################################################
 # Flex/Bison rules

bazel/check_deps/check_non_test_cc_deps.py

@@ -56,7 +56,7 @@ for dep in deps:
 
         # The rest of LLVM, LLD, and Clang themselves are safe to depend on.
         continue
-    if repo in ("@llvm_terminfo", "@llvm_zlib", "@zlib"):
+    if repo in ("@llvm_terminfo", "@llvm_zlib", "@llvm_zstd"):
         # These are stubs wrapping system libraries for LLVM. They aren't
         # distributed and so should be fine.
         continue