[libcxx] [llvm] [libc++][CI] Updates to Clang 19. (PR #85301)

[Louis Dionne via llvm-commits]

================
@@ -38,11 +38,11 @@ env:
   # LLVM POST-BRANCH bump version
   # LLVM POST-BRANCH add compiler test for ToT - 1, e.g. "Clang 17"
   # LLVM RELEASE bump remove compiler ToT - 3, e.g. "Clang 15"
-  LLVM_HEAD_VERSION: "18"   # Used compiler, update POST-BRANCH.
-  LLVM_PREVIOUS_VERSION: "17"
-  LLVM_OLDEST_VERSION: "16"
+  LLVM_HEAD_VERSION: "19"   # Used compiler, update POST-BRANCH.
+  LLVM_PREVIOUS_VERSION: "18"
+  LLVM_OLDEST_VERSION: "17"
   GCC_STABLE_VERSION: "13"
-  LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-18"
+  LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-17"
----------------
ldionne wrote:

```suggestion
  LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-19"
```

https://github.com/llvm/llvm-project/pull/85301