cmake, ci: Bump clang version to 15

[hebasto]

See bitcoin#29165.

[fanquake]

Is this being moved away from our minimum on purpose? Otherwise I don't see why this is needed, or how bitcoin#29859 is related to unreachable code warnings.

[hebasto]

Is this being moved away from our minimum on purpose?

These CI tasks are to test CMake-specific points of the build system rather than compiler support in general. And .github/workflows/cmake.yml won't go to the master branch.

Otherwise I don't see why this is needed

To keep CI config simpler.

how bitcoin#29859 is related to unreachable code warnings.

bitcoin#29859 is related to compiling with clang-15. And unreachable code warnings are observable on clang-14.

[vasild]

FWIW on latest master @ 2a07c46 with clang 19 I see:

clang++-devel -std=c++20 -DHAVE_CONFIG_H -I. -I../src/config  -DDEBUG -DDEBUG_LOCKORDER -DDEBUG_LOCKCONTENTION -DRPC_DOC_CHECK -DABORT_ON_FAILED_ASSUME  -DHAVE_BUILD_INFO -DPROVIDE_FUZZ_MAIN_FUNCTION -I. -I./minisketch/include -I./secp256k1/include -I./univalue/include  -isystem /usr/local/include -DBOOST_MULTI_INDEX_DISABLE_SERIALIZATION -isystem /usr/local/include  -I/usr/local/include -g -O2 -O0 -g3 -ftrapv -Wstack-protector -fstack-protector-all -fcf-protection=full -fstack-clash-protection -Wall -Wextra -Wgnu -Wformat -Wformat-security -Wvla -Wshadow-field -Wthread-safety -Wloop-analysis -Wredundant-decls -Wunused-member-function -Wdate-time -Wconditional-uninitialized -Woverloaded-virtual -Wsuggest-override -Wimplicit-fallthrough -Wunreachable-code -Wdocumentation -Wno-unused-parameter -Wno-self-assign -Werror   -fPIE  -MT test/test_bitcoin-util_threadnames_tests.o -MD -MP -MF test/.deps/test_bitcoin-util_threadnames_tests.Tpo -c -o test/test_bitcoin-util_threadnames_tests.o `test -f 'test/util_threadnames_tests.cpp' || echo './'`test/util_threadnames_tests.cpp

test/util_threadnames_tests.cpp:62:35: error: code will never be executed [-Werror,-Wunreachable-code]
   62 |     std::set<std::string> names = RenameEnMasse(100);
      |                                   ^~~~~~~~~~~~~

[fanquake]

I'm assuming that's on FreeBSD? If so, I assume it's because we currently outright disable thread local.

[vasild]

@fanquake, yes to both.

[hebasto]

Rebased to resolve conflicts.

[hebasto]

I've updated PR title and description in the light of the merged bitcoin#29165.

[hebasto]

cc @m3dwards @vasild @pablomartin4btc

[m3dwards]

utACK 8fd40d4