yvals_core.h: Improve "error STL1003: Unexpected compiler, expected C++ compiler." like GH 4020.

Then we can move `_STL_STRINGIZE` down to its point of use, near our other macros.

```
    D:\GitHub\STL\out\x64>type woof.c
    #include <vector>

    D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /c woof.c
    woof.c
    D:\GitHub\STL\out\x64\out\inc\yvals_core.h(23): fatal error C1189: #error:  error STL1003: Unexpected compiler, expected C++ compiler.
```

Author: StephanTLavavej

Diff for: stl/inc/yvals_core.h

@@ -18,12 +18,9 @@
 
 #if _STL_COMPILER_PREPROCESSOR
 
-// This does not use `_EMIT_STL_ERROR`, as it needs to be checked before we include anything else.
-#define _STL_STRINGIZE_(S) #S
-#define _STL_STRINGIZE(S)  _STL_STRINGIZE_(S)
+// This does not use `_EMIT_STL_ERROR`, as it is checking the language itself.
 #ifndef __cplusplus
-#pragma message(__FILE__ "(" _STL_STRINGIZE(__LINE__) "): STL1003: Unexpected compiler, expected C++ compiler.")
-#error Error in C++ Standard Library usage
+#error error STL1003: Unexpected compiler, expected C++ compiler.
 #endif // !defined(__cplusplus)
 
 // Implemented unconditionally:
@@ -504,6 +501,9 @@
 #include <vcruntime.h>
 #include <xkeycheck.h> // The _HAS_CXX tags must be defined before including this.
 
+#define _STL_STRINGIZE_(S) #S
+#define _STL_STRINGIZE(S)  _STL_STRINGIZE_(S)
+
 // Note that _STL_PRAGMA is load-bearing;
 // it still needs to exist even once CUDA and ICC support _Pragma.
 #if defined(__CUDACC__) || defined(__INTEL_COMPILER)