VS: Restore compilation of '.C' sources as C++

Refactoring in commit 3b547e2e4b (VS: Simplify logic adding source file
C/C++ language flag to MSVC, 2020-05-15, v3.18.0-rc1~139^2~1) failed to
account for MSVC's treatment of `.C` extensions as C.  Add this special
case to the logic to restore use of `-TP` for `.C` extensions.

Fixes: #20822

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2321,11 +2321,14 @@ void cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
   }
 
   // Force language if the file extension does not match.
+  // Note that MSVC treats the upper-case '.C' extension as C and not C++.
+  std::string const ext = sf.GetExtension();
+  std::string const extLang = ext == "C"
+    ? "C"
+    : this->GlobalGenerator->GetLanguageFromExtension(ext.c_str());
   std::string lang = this->LocalGenerator->GetSourceFileLanguage(sf);
   const char* compileAs = 0;
-  if (lang !=
-      this->GlobalGenerator->GetLanguageFromExtension(
-        sf.GetExtension().c_str())) {
+  if (lang != extLang) {
     if (lang == "CXX") {
       // force a C++ file type
       compileAs = "CompileAsCpp";

Tests/CxxOnly/CMakeLists.txt

@@ -5,7 +5,7 @@ set(CMAKE_DEBUG_POSTFIX "_test_debug_postfix")
 if(WIN32)
   set(EXTRA_SRCS test.CPP)
 endif()
-add_library(testcxx1.my STATIC libcxx1.cxx ${EXTRA_SRCS})
+add_library(testcxx1.my STATIC libcxx1.cxx test.C ${EXTRA_SRCS})
 add_library(testcxx2 SHARED libcxx2.cxx)
 add_executable (CxxOnly cxxonly.cxx)
 target_link_libraries(CxxOnly testcxx1.my testcxx2)

Tests/CxxOnly/cxxonly.cxx

@@ -1,5 +1,6 @@
 #include "libcxx1.h"
 #include "libcxx2.h"
+extern int testC;
 #ifdef _MSC_VER
 extern int testCPP;
 #endif
@@ -8,6 +9,7 @@ extern int testCPP;
 
 int main()
 {
+  testC = 1;
 #ifdef _MSC_VER
   testCPP = 1;
 #endif

Tests/CxxOnly/test.C

@@ -0,0 +1 @@
+int testC;