Change in coreboot[master]: tests: Fix function mocking for clang

[Jakub Czapiga (Code Review)]

Attention is currently required from: Julius Werner.
Jakub Czapiga has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/57009 )

Change subject: tests: Fix function mocking for clang
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Patch Set 1:

(1 comment)

File tests/Makefile.inc:

https://review.coreboot.org/c/coreboot/+/57009/comment/f165386b_4caae9c9 
PS1, Line 60: TEST_CFLAGS += $(if $(filter clang,$(shell $(HOSTCC) --version)),,-Werror)
In the llvm-project at clang/lib/Sema/Sema.cpp:1167 [1] there is a check that generates those warnings. We could add a new diagnostic flag e.g. "-Wno-weak-identifier-undeclared" and add check:
> if (!Diags.isIgnored(diag::warn_weak_identifier_undeclared, SourceLocation()))
>      Diag(WeakID.second.getLocation(), diag::warn_weak_identifier_undeclared)
>          << WeakID.first;

Next we would have to add commandline flag for it to:
lang/include/clang/Basic/DiagnosticGroups.td:
> def WarnWeakIdentifierUndeclared : DiagGroup<"warn_weak_identifier_undeclared">

clang/include/clang/Basic/DiagnosticSemaKinds.td:
> def warn_weak_identifier_undeclared : Warning<
>    "weak identifier %0 never declared">,
>    InGroup<WarnWeakIdentifierUndeclared>;

Probably there will also be a need for new tests or updates of old ones. I am sure, we have to remove warn_weak_identifier_undeclared from clang/test/Misc/warning-flags.c

[1] https://github.com/llvm/llvm-project/blob/acc58504952f0e2ca48a44d868f684e3b1...