CI: Use LLVM 15 so llvm-nm works in opaque pointer mode.

mk/cargo.sh

@@ -43,7 +43,7 @@ for arg in $*; do
 done
 
 # See comments in install-build-tools.sh.
-llvm_version=14
+llvm_version=15
 
 case $target in
    aarch64-linux-android)

mk/check-symbol-prefixes.sh

@@ -17,7 +17,7 @@
 set -eux -o pipefail
 IFS=$'\n\t'
 
-llvm_version=14
+llvm_version=15
 
 for arg in $*; do
   case $arg in

mk/install-build-tools.sh

@@ -85,7 +85,7 @@ esac
 if [ -n "$use_clang" ]; then
   # https://github.com/rustls/rustls/pull/1009 upgraded Rust's LLVM version to
   # 14
-  llvm_version=14
+  llvm_version=15
   sudo apt-key add mk/llvm-snapshot.gpg.key
   sudo add-apt-repository "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-$llvm_version main"
   sudo apt-get update