bump clang-format to clang-15

clang 11 was released in 2007, and the last variant of Ubuntu with the
clang-format-11 package is 22.04.

This bumps the clang-format used for the formatting check to clang 15,
available with Ubuntu 22.04 and 24.04.

Author: kroening

Diff for: .github/workflows/pull-request-check-clang-format.sh

@@ -7,7 +7,7 @@ set -e
 echo "Pull request's base branch is: ${BASE_BRANCH}"
 echo "Pull request's merge branch is: ${MERGE_BRANCH}"
 echo "Pull request's source branch is: ${GITHUB_HEAD_REF}"
-clang-format-11 --version
+clang-format-15 --version
 
 # The checkout action leaves us in detatched head state. The following line
 # names the checked out commit, for simpler reference later.
@@ -26,7 +26,7 @@ echo "Checking for formatting errors introduced since $MERGE_BASE"
 
 # Do the checking. "eval" is used so that quotes (as inserted into $EXCLUDES
 # above) are not interpreted as parts of file names.
-eval git-clang-format-11 --binary clang-format-11 $MERGE_BASE -- $EXCLUDES
+eval git-clang-format-15 --binary clang-format-15 $MERGE_BASE -- $EXCLUDES
 git diff > formatted.diff
 if [[ -s formatted.diff ]] ; then
   echo 'Formatting error! The following diff shows the required changes'

Diff for: .github/workflows/syntax-checks.yaml

@@ -6,7 +6,7 @@ on:
 jobs:
   # This job takes approximately 1 minute
   check-clang-format:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
         with:
@@ -19,8 +19,8 @@ jobs:
           DEBIAN_FRONTEND: noninteractive
         run: |
           sudo apt-get update
-          sudo apt-get install --no-install-recommends -yq clang-format-11
-      - name: Check updated lines of code match clang-format-11 style
+          sudo apt-get install --no-install-recommends -yq clang-format-15
+      - name: Check updated lines of code match clang-format-15 style
         env:
           BASE_BRANCH: ${{ github.base_ref }}
           MERGE_BRANCH: ${{ github.ref }}

Diff for: CODING_STANDARD.md

@@ -340,14 +340,13 @@ Now, you can commit and push the formatting fixes.
 To avoid waiting until you've made a PR to find formatting issues, you can
 install clang-format locally and run it against your code as you are working.
 
-Different versions of clang-format have slightly different behaviors. CBMC uses
-clang-format-11 as it is available the repositories for Ubuntu 20.04 and
-Homebrew.
-To install on a Unix-like system, try installing using the system package
-manager:
+Different versions of clang-format have slightly different behaviors.  CBMC
+uses clang-format-15, or later, available in the standard Ubuntu 24.04 and
+Homebrew repositories.  To install on a Unix-like system, try installing
+using the system package manager:
 ```
-apt-get install clang-format-11  # Run this on Ubuntu, Debian etc.
-brew install clang-format@11     # Run this on a Mac with Homebrew installed
+apt-get install clang-format     # Run this on Ubuntu, Debian etc.
+brew install clang-format        # Run this on a Mac with Homebrew installed
 ```
 
 If your platform doesn't have a package for clang-format, you can download a