try_compile: Restore toleration of toolchain file setting internal variable

Prior to commit 243462fafe (try_compile: Tolerate compile features from
non-enabled languages, 2026-05-18, v4.4.0-rc1~84^2) we accidentally
tolerated toolchain files setting `CMAKE_<LANG>_COMPILER_LOADED`.
Meson accidentally relied on this, so restore toleration of it.

Fixes: #27971

Source/cmGlobalGenerator.cxx

@@ -850,8 +850,11 @@ void cmGlobalGenerator::EnableLanguage(
       this->SetLanguageEnabled("NONE", mf);
       continue;
     }
-    std::string loadedLang = cmStrCat("CMAKE_", lang, "_COMPILER_LOADED");
-    if (!mf->GetDefinition(loadedLang)) {
+    // Compiler information may have already been detected and saved.
+    // Load it if we have not enabled the language anywhere yet, or
+    // if we have not loaded compiler information in this directory.
+    if (!this->GetLanguageEnabled(lang) ||
+        !mf->GetDefinition(cmStrCat("CMAKE_", lang, "_COMPILER_LOADED"))) {
       fpath = cmStrCat(rootBin, "/CMake", lang, "Compiler.cmake");
 
       // If the existing build tree was already configured with this

Tests/RunCMake/ToolchainFile/RunCMakeTest.cmake

@@ -14,6 +14,7 @@ run_cmake_toolchain(LinkFlagsInit)
 run_cmake_toolchain(CMP0126-NEW)
 run_cmake_toolchain(CMP0126-OLD)
 run_cmake_toolchain(CMP0126-WARN)
+run_cmake_toolchain(SetCompilerLoaded)
 run_cmake_toolchain(SetCrossCompiling)
 
 function(run_IncludeDirectories)

Tests/RunCMake/ToolchainFile/SetCompilerLoaded-toolchain.cmake

@@ -0,0 +1 @@
+set(CMAKE_C_COMPILER_LOADED 1)

Tests/RunCMake/ToolchainFile/SetCompilerLoaded.cmake

@@ -0,0 +1,5 @@
+enable_language(C)
+try_compile(SHOULD_PASS SOURCE_FROM_CONTENT "main.c" "int main(void) { return 0; }")
+if(NOT SHOULD_PASS)
+  message(FATAL_ERROR "try_compile failed but should have passed!")
+endif()