[libunwind] Bump to CXX_STANDARD 17 (#75986)

libunwind uses C-style and low-level C++, so the language standard
doesn't matter that much, but bumping to C++17 aligns with the rest of
LLVM and enables some features that would cause pedantic warnings in
C++11 (e.g. -Wc++17-attribute-extensions for [[fallthrough]]/
[[nodiscard]]/[[maybe_unused]]). (Contributors might use these features
unaware of the pedantic warnings).

Suggested-by: Christopher Di Bella <[email protected]>

Author: MaskRay

libunwind/src/CMakeLists.txt

@@ -154,7 +154,7 @@ target_link_libraries(unwind_shared_objects PUBLIC "${LIBUNWIND_ADDITIONAL_LIBRA
 set_target_properties(unwind_shared_objects
   PROPERTIES
     CXX_EXTENSIONS OFF
-    CXX_STANDARD 11
+    CXX_STANDARD 17
     CXX_STANDARD_REQUIRED ON
     COMPILE_FLAGS "${LIBUNWIND_COMPILE_FLAGS}"
 )
@@ -194,7 +194,7 @@ target_link_libraries(unwind_static_objects PUBLIC "${LIBUNWIND_ADDITIONAL_LIBRA
 set_target_properties(unwind_static_objects
   PROPERTIES
     CXX_EXTENSIONS OFF
-    CXX_STANDARD 11
+    CXX_STANDARD 17
     CXX_STANDARD_REQUIRED ON
     COMPILE_FLAGS "${LIBUNWIND_COMPILE_FLAGS}"
 )