bump clang-format to clang-15

[kroening]

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 24.04.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.95%. Comparing base (36b2335) to head (9030c33).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8561      +/-   ##
===========================================
+ Coverage    78.79%   78.95%   +0.15%     
===========================================
  Files         1730     1730              
  Lines       199160   198759     -401     
  Branches     18320    18314       -6     
===========================================
  Hits        156938   156938              
+ Misses       42222    41821     -401     

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[TGWDB]

I'm not an expert on brew, but this seems like it will break in future when the default is upgraded (where as all the other places and previous code are explicit about the version).

[tautschnig]

It seems Brew only has versioned variants for clang-format 8 and clang-format 11, while the latest one will install clang-format 19.

[tautschnig]

Let's just say "clang-format 11 or later" -- our clang-format configuration file does not use options that were introduced in any version greater than 11, so this should be just fine.

[tautschnig]

It seems Brew only has versioned variants for clang-format 8 and clang-format 11, while the latest one will install clang-format 19.

[tautschnig]

Suggested change

	clang-format-15, available in the standard Ubuntu 24.04 and Homebrew
	repositories. To install on a Unix-like system, try installing using the
	system package manager:
	clang-format-11 or later, available in the standard Ubuntu and Homebrew
	repositories. To install on a Unix-like system, try installing using the
	system package manager:

[tautschnig]

Suggested change

	apt-get install clang-format-15 # Run this on Ubuntu, Debian etc.
	apt-get install clang-format # Run this on Ubuntu, Debian etc.