VS: Restore toleration of target-wide -TP flag with MSVC

Since commit 3b547e2e4b (VS: Simplify logic adding source file C/C++
language flag to MSVC, 2020-05-15, v3.18.0-rc1~139^2~1) we only add a
per-source language selection flag when the source file extension does
not match the compiler's default.  This approach breaks when a project
adds a target-wide `-TP` flag.

Although such projects likely did not work with non-VS generators, we
did support them before in Visual Studio generators.  Add a special case
to tolerate such flags again.

Fixes: #21005

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2884,6 +2884,12 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
     clOptions.RemoveFlag("SpectreMitigation");
   }
 
+  // Remove any target-wide -TC or -TP flag added by the project.
+  // Such flags are unnecessary and break our model of language selection.
+  if (langForClCompile == "C" || langForClCompile == "CXX") {
+    clOptions.RemoveFlag("CompileAs");
+  }
+
   this->ClOptions[configName] = std::move(pOptions);
   return true;
 }

Tests/SetLang/CMakeLists.txt

@@ -8,3 +8,10 @@ add_executable(SetLang bar.c)
 set_source_files_properties(foo.c bar.c PROPERTIES LANGUAGE CXX)
 target_link_libraries(SetLang foo)
 set_target_properties(SetLang PROPERTIES LINKER_LANGUAGE CXX)
+
+# VS generators historically tolerated target-wide -TP flags added
+# by project code, so cover that case to preserve the behavior.
+if(CMAKE_GENERATOR MATCHES "^Visual Studio" AND "x${CMAKE_C_COMPILER_ID}" STREQUAL "xMSVC")
+  add_library(stay stay_c.c stay_cxx.cxx)
+  set_property(TARGET stay PROPERTY COMPILE_OPTIONS "-TP")
+endif()

Tests/SetLang/stay_c.c

@@ -0,0 +1,8 @@
+#ifdef __cplusplus
+#  error C source incorrectly compiled as C++
+#endif
+
+int stay_c(void)
+{
+  return 0;
+}

Tests/SetLang/stay_cxx.cxx

@@ -0,0 +1,8 @@
+#ifndef __cplusplus
+#  error C++ source incorrectly compiled as C
+#endif
+
+int stay_cxx()
+{
+  return 0;
+}