[libcxx] Only add -GR- option to MSVC or clang-cl builds

[DavidSpickett]

Summary

Previously we added both -GR- and -fno-rtti if RTTI was disabled.
When building with clang 16.x, that caused this error in part of the build:

clang-16: error: argument unused during compilation: '-G R-' [-Werror,-Wunused-command-line-argument]

I think the strange message is because clang is seeing R- as the argument
to -G, which is a valid clang option.

-GR- is an alternate syntax for the /GR- option for MSVC
(the dash means disable RTTI):
https://learn.microsoft.com/en-us/cpp/build/reference/gr-enable-run-time-type-information?view=msvc-170

This error is sort of fixed by cd18efb61d759405956dbd30e4b5f2720d8e1783
but not intentionally. Also, we'd have to wait for 17.x to benefit from that.

The proper fix here is to only add -GR- if we are building with MSVC
or the MSVC-like clang-cl, and add -fno-rtti if not.

[DavidSpickett]

I've confirmed this works with the build proposed in https://reviews.llvm.org/D154246, no need to disable the warning.

[simon_tatham]

Looks good to me – but then I would say that, it was my suggestion :-)

Good catch on finding LIBCXX_TARGETING_CLANG_CL as well as LIBCXX_TARGETING_MSVC. I'd only spotted the latter.

[michaelplatings]

Thanks for this, LGTM.

[mstorsjo]

libcxx/CMakeLists.txt

572

I'm late to the party here, but AFAIK we don't need to use these very libcxx specific cmake defines here - if (MSVC) is just enough as well; the cmake MSVC define is set for MSVC proper or any commandline compatible environment (icl, clang-cl).

[DavidSpickett]

libcxx/CMakeLists.txt

572

https://reviews.llvm.org/D154698

libcxx/CMakeLists.txt

else()
     target_add_compile_flags_if_supported(${target} PUBLIC -EHs- -EHa-)
     target_add_compile_flags_if_supported(${target} PUBLIC -fno-exceptions)
   endif()
 endfunction()
 
 # RTTI flags ==================================================================
 function(cxx_add_rtti_flags target)
   if (NOT LIBCXX_ENABLE_RTTI)
+    if (LIBCXX_TARGETING_CLANG_CL OR LIBCXX_TARGETING_MSVC)
       target_add_compile_flags_if_supported(${target} PUBLIC -GR-)
+    else()
       target_add_compile_flags_if_supported(${target} PUBLIC -fno-rtti)
     endif()
+  endif()
 endfunction()
 
 # Threading flags =============================================================
 if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY AND LIBCXX_ENABLE_SHARED)
   # Need to allow unresolved symbols if this is to work with shared library builds
   if (APPLE)
     add_link_flags("-undefined dynamic_lookup")
   else()