build: update clang v21, fix warnings

- `src/nvim/ex_cmds_defs.h`: use "U" instead of "u" per
  `readability-uppercase-literal-suffix`

.github/scripts/install_deps.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+set -e
 
 while (($# > 0)); do
   case $1 in
@@ -17,7 +18,7 @@ if [[ $OS == Linux ]]; then
 
   if [[ $CC == clang ]]; then
     DEFAULT_CLANG_VERSION=$(echo |  clang -dM -E - | grep __clang_major | awk '{print $3}')
-    CLANG_VERSION=19
+    CLANG_VERSION=21
     if ((DEFAULT_CLANG_VERSION >= CLANG_VERSION)); then
       echo "Default clang version is $DEFAULT_CLANG_VERSION, which is equal or larger than wanted version $CLANG_VERSION. Aborting!"
       exit 1
@@ -26,6 +27,8 @@ if [[ $OS == Linux ]]; then
     wget https://apt.llvm.org/llvm.sh
     chmod +x llvm.sh
     sudo ./llvm.sh $CLANG_VERSION
+    # Alternatively could use `./llvm.sh … all` above, but that is heavy/slow.
+    sudo apt-get install -y clang-tidy-$CLANG_VERSION
     sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-$CLANG_VERSION 100
     sudo update-alternatives --set clang /usr/bin/clang-$CLANG_VERSION
     sudo update-alternatives --install /usr/bin/clang-tidy clang-tidy /usr/bin/clang-tidy-$CLANG_VERSION 100