Demangling: degrade better with VS2017

`_Pragma` support was added to VS with VS2019 U6, version 16.6, toolset
14.26.  Add a conditional to avoid the use of `_Pragma` prior to that
version.

Author: compnerd

include/swift/Demangling/ManglingMacros.h

@@ -17,10 +17,14 @@
 #define MANGLE_AS_STRING(M) STRINGIFY_MANGLING(M)
 
 /// The mangling prefix for the new mangling.
+#if !defined(_MSC_VER) || _MSC_VER-0 >= 1926
 _Pragma("clang diagnostic push")
 _Pragma("clang diagnostic ignored \"-Wdollar-in-identifier-extension\"")
+#endif
 #define MANGLING_PREFIX $s
+#if !defined(_MSC_VER) || _MSC_VER-0 >= 1926
 _Pragma("clang diagnostic pop")
+#endif
 
 #define MANGLING_PREFIX_STR MANGLE_AS_STRING(MANGLING_PREFIX)