[cmake] Enable -Werror=return-type

[kastiglione]

Summary

Turn -Wreturn-type into an error.

This is currently used by libcxx, libcxxabi, and libunwind, and would be a good default
for all of llvm. I'm not aware of any cases where this shouldn't be an error. This
ensures different build configs, merges, and downstream branches catch issues sooner.

[JDevlieghere]

LGTM

[kastiglione]

Reverted because some versions of gcc can emit -Wreturn-type warnings on functions that have exhaustive switches over enums. A fix is to use llvm_unreachable but that's not currently done everywhere (and also seems like a poor/unnecessary practice).

[xbolva00]

Can you either enable it for Clang or fix llvm codebase so GCC is ok with it?

[kastiglione]

@xbolva00 yes I plan to do that. I'll quickly check into the gcc situation first, and but may just re-enable it for clang.

llvm/cmake/modules/HandleLLVMOptions.cmake

CHECK_C_SOURCE_COMPILES("// \\\\\\n//\\nint main() {return 0;}"
                           C_WCOMMENT_ALLOWS_LINE_WRAP)
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
   if (NOT C_WCOMMENT_ALLOWS_LINE_WRAP)
     append("-Wno-comment" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
 
   # Enable -Wstring-conversion to catch misuse of string literals.
   add_flag_if_supported("-Wstring-conversion" STRING_CONVERSION_FLAG)
+
+  # Enable -Werror=return-type if available, to catch functions that contain
+  # control paths that do not return a value.
+  add_flag_if_supported("-Werror=return-type" WERROR_RETURN_TYPE)
 endif (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE AND NOT LLVM_ENABLE_WARNINGS)
   append("-w" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 endif()
 
 macro(append_common_sanitizer_flags)
   if (NOT MSVC)