Genex: Enable COMPILE_LANGUAGE for COMPILE_DEFINITIONS with VS and Xcode

The set of compile flags used for a target's C and C++ sources is based
on the linker language.  By default this is always the C++ flags if any
C++ sources appear in the target, and otherwise the C flags.  Therefore
we can define the `COMPILE_LANGUAGE` generator expression in
`COMPILE_DEFINITIONS` to match the selected language.

This is not exactly the same as for other generators, but is the best VS
and Xcode can do.  It is also sufficient for many use cases since the
set of definitions for C and C++ is frequently similar but may be
distinct from those for other languages like CUDA.

Issue: #17435

Tests/CudaOnly/WithDefs/CMakeLists.txt

@@ -32,8 +32,8 @@ add_executable(CudaOnlyWithDefs ${main})
 
 target_compile_options(CudaOnlyWithDefs
   PRIVATE
-    -DCOMPILE_LANG_$<COMPILE_LANGUAGE>
-    -DLANG_IS_CUDA=$<COMPILE_LANGUAGE:CUDA>
+    -DFLAG_COMPILE_LANG_$<COMPILE_LANGUAGE>
+    -DFLAG_LANG_IS_CUDA=$<COMPILE_LANGUAGE:CUDA>
     -Xcompiler=-DHOST_DEFINE
     $<$<CONFIG:DEBUG>:$<BUILD_INTERFACE:${debug_compile_flags}>>
   )
@@ -41,6 +41,8 @@ target_compile_options(CudaOnlyWithDefs
 target_compile_definitions(CudaOnlyWithDefs
   PRIVATE
     $<$<CONFIG:RELEASE>:$<BUILD_INTERFACE:${release_compile_defs}>>
+    -DDEF_COMPILE_LANG_$<COMPILE_LANGUAGE>
+    -DDEF_LANG_IS_CUDA=$<COMPILE_LANGUAGE:CUDA>
   )
 
 if(APPLE)

Tests/CudaOnly/WithDefs/main.notcu

@@ -10,16 +10,28 @@
 #error "PACKED_DEFINE not defined!"
 #endif
 
-#ifndef COMPILE_LANG_CUDA
-#error "COMPILE_LANG_CUDA not defined!"
+#ifndef FLAG_COMPILE_LANG_CUDA
+#error "FLAG_COMPILE_LANG_CUDA not defined!"
 #endif
 
-#ifndef LANG_IS_CUDA
-#error "LANG_IS_CUDA not defined!"
+#ifndef FLAG_LANG_IS_CUDA
+#error "FLAG_LANG_IS_CUDA not defined!"
 #endif
 
-#if !LANG_IS_CUDA
-#error "Expected LANG_IS_CUDA"
+#if !FLAG_LANG_IS_CUDA
+#error "Expected FLAG_LANG_IS_CUDA"
+#endif
+
+#ifndef DEF_COMPILE_LANG_CUDA
+#error "DEF_COMPILE_LANG_CUDA not defined!"
+#endif
+
+#ifndef DEF_LANG_IS_CUDA
+#error "DEF_LANG_IS_CUDA not defined!"
+#endif
+
+#if !DEF_LANG_IS_CUDA
+#error "Expected DEF_LANG_IS_CUDA"
 #endif
 
 static __global__ void DetermineIfValidCudaDevice()