The code was "modernized" (the term is too ambiguous and subjective to take it seriously) and thus has slightly diverged from the way it's presented in the post. Original tree that corresponds to the text can be browsed here.

Clang-based standalone tool that diagnoses the following cases of defining functions:

• completely unused functions declared as extern;
• functions declared as extern which are used only locally and thus can be made static.

Input:

static void firstStatic(void);
static void secondStatic(void);

void firstExtern(void);

extern void secondExtern(void);

static void firstStatic(void)  { }
static void secondStatic(void) { }

void firstExtern(void)  { }
void secondExtern(void) { }

int
main(void)
{
    firstExtern();
    secondStatic();
    return 0;
}

Run command:

bin/unused-funcs main.c --

Output (paths are truncated):

.../main.c:20:firstExtern:can be made static
.../main.c:25:secondExtern:unused

1. Clone the repository to llvm/tools/clang/tools/extra.
2. Add add_subdirectory(unused-funcs) to CMakeLists.txt in the same directory.
3. Run ninja unused-funcs inside your LLVM's build/ directory.