CMakeDetermineCompilerId: extract C++ standard library impl

This is needed for C++ modules because `import std` support may differ
based on the standard library implementation. Extract the information as
necessary.

Modules/CMakeDetermineCompilerId.cmake

@@ -312,6 +312,29 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
     endif ()
   endif ()
 
+  set(CMAKE_${lang}_STANDARD_LIBRARY "")
+  if ("x${lang}" STREQUAL "xCXX" AND
+      EXISTS "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${lang}-DetectStdlib.h" AND
+      "x${CMAKE_${lang}_COMPILER_ID}" STREQUAL "xClang" AND
+      "x${CMAKE_${lang}_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU")
+    # See #20851 for a proper abstraction for this.
+    execute_process(
+      COMMAND "${CMAKE_${lang}_COMPILER}"
+        ${CMAKE_${lang}_COMPILER_ID_ARG1}
+        ${CMAKE_CXX_COMPILER_ID_FLAGS_LIST}
+        -E
+        -x c++-header
+        "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${lang}-DetectStdlib.h"
+        -o - # Write to stdout.
+      OUTPUT_VARIABLE _lang_stdlib_out
+      ERROR_VARIABLE _lang_stdlib_err
+      RESULT_VARIABLE _lang_stdlib_res
+      ERROR_STRIP_TRAILING_WHITESPACE)
+    if (_lang_stdlib_res EQUAL 0)
+      string(REGEX REPLACE ".*CMAKE-STDLIB-DETECT: (.+)\n.*" "\\1" "CMAKE_${lang}_STANDARD_LIBRARY" "${_lang_stdlib_out}")
+    endif ()
+  endif ()
+
   # Display the final identification result.
   if(CMAKE_${lang}_COMPILER_ID)
     if(CMAKE_${lang}_COMPILER_VERSION)
@@ -355,6 +378,7 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
   set(CMAKE_${lang}_COMPILER_PRODUCED_OUTPUT "${COMPILER_${lang}_PRODUCED_OUTPUT}" PARENT_SCOPE)
   set(CMAKE_${lang}_COMPILER_PRODUCED_FILES "${COMPILER_${lang}_PRODUCED_FILES}" PARENT_SCOPE)
   set(CMAKE_${lang}_COMPILER_CLANG_RESOURCE_DIR "${CMAKE_${lang}_COMPILER_CLANG_RESOURCE_DIR}" PARENT_SCOPE)
+  set(CMAKE_${lang}_STANDARD_LIBRARY "${CMAKE_${lang}_STANDARD_LIBRARY}" PARENT_SCOPE)
 endfunction()
 
 include(CMakeCompilerIdDetection)

Modules/CXX-DetectStdlib.h

@@ -0,0 +1,10 @@
+#include <version>
+// clang-format off
+#if defined(_LIBCPP_VERSION)
+CMAKE-STDLIB-DETECT: libc++
+#elif defined(__GLIBCXX__)
+CMAKE-STDLIB-DETECT: libstdc++
+#else
+CMAKE-STDLIB-DETECT: UNKNOWN
+#endif
+    // clang-format on