Add a comment for why Demangler::isDigit isn't just std::isdigit.

Author: jckarter

Diff for: include/swift/Basic/Demangle.h

@@ -407,6 +407,10 @@ class DemanglerPrinter {
   std::string &Stream;
 };
 
+/// Is a character considered a digit by the demangling grammar?
+///
+/// Yes, this is equivalent to the standard C isdigit(3), but some platforms
+/// give isdigit suboptimal implementations.
 static inline bool isDigit(int c) {
   return c >= '0' && c <= '9';
 }