[clang-tidy] llvmlibc-inline-function-decl erroneously reports on lambda functions

[jhuber6]

The llvmlibc-inline-function-decl currently erroneously reports when defining a lambda function. The reason LLVM's libc reports on these variables so that we can ensure functions in headers line up when compiled differently for different TUs. However, the functions emitted from lambda values will have internal linkage, so they will be unique to each TU they are implemented in. Here is a reproducer.

namespace __llvm_libc {

void foo(void (*fn)(void));

void bar() {
  foo([]() {});
}

} // namespace __llvm_libc

$ clang-tidy test.h --checks=-'*,llvmlibc*'               
8 warnings generated.
test.h:6:7: warning: '__invoke' must be tagged with the LIBC_INLINE macro; the macro should be placed at the beginning of the declaration [llvmlibc-inline-function-decl]
  foo([]() {});
      ^
test.h:6:7: warning: 'operator void (*)()' must be tagged with the LIBC_INLINE macro; the macro should be placed at the beginning of the declaration [llvmlibc-inline-function-decl]
test.h:6:7: warning: '~(lambda at /home/jhuber/Documents/llvm/llvm-project/build/test.h:6:7)' must be tagged with the LIBC_INLINE macro; the macro should be placed at the beginning of the declaration [llvmlibc-inline-function-decl]
test.h:6:10: warning: 'operator()' must be tagged with the LIBC_INLINE macro; the macro should be placed at the beginning of the declaration [llvmlibc-inline-function-decl]
  foo([]() {});
         ^
Suppressed 4 warnings (4 with check filters).

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