[libunwind] Fix build with -Wunused-function

[smeenai]

Summary

https://reviews.llvm.org/D144252 removed -Wno-unused-function from the
libunwind build, but we have an unused function when you're building for
armv7 without assertions. Mark that function as possibly unused to avoid
the warning, and mark the parameter as a const pointer while I'm here to
make it clear that nothing is modified by a debugging function.

[philnik]

I would just mark it [[maybe_unused]]. It's not like this is costly to compile.

[smeenai]

In D156496#4540878, @philnik wrote:

I would just mark it [[maybe_unused]]. It's not like this is costly to compile.

libunwind builds with C++11 (https://github.com/llvm/llvm-project/blob/2f976b921071cf4349175554f690798a0d2b81f3/libunwind/src/CMakeLists.txt#L140). Dunno if that's intentional or just an oversight. Both gcc and Clang seem to support [[maybe_unused]] in C++11 as an extension anyway, but I could use [[gnu::unused]] if we want to avoid -pedantic warnings.

[smeenai]

Use [[gnu::unused]] instead

[philnik]

I don't think there is any reason that we use C++11 for libunwind, but that is a different project. LGTM.

libunwind/src/Unwind-EHABI.cpp

_Unwind_GetLanguageSpecificData(struct _Unwind_Context *context) {
   if (__unw_get_proc_info(cursor, &frameInfo) == UNW_ESUCCESS)
     result = (uintptr_t)frameInfo.lsda;
   _LIBUNWIND_TRACE_API(
       "_Unwind_GetLanguageSpecificData(context=%p) => 0x%llx",
       static_cast<void *>(context), (long long)result);
   return result;
 }
 
-static uint64_t ValueAsBitPattern(_Unwind_VRS_DataRepresentation representation,
-                                  void* valuep) {
+// Only used in _LIBUNWIND_TRACE_API, which is a no-op when assertions are
+// disabled.
+[[gnu::unused]] static uint64_t
+ValueAsBitPattern(_Unwind_VRS_DataRepresentation representation,
+                  const void *valuep) {
   uint64_t value = 0;
   switch (representation) {
     case _UVRSD_UINT32:
     case _UVRSD_FLOAT:
       memcpy(&value, valuep, sizeof(uint32_t));
       break;
 
     case _UVRSD_VFPX: