Use sizeof for buffer lengths in snprintf

Rather than duplicating the constant value, use the `sizeof` operator to have
the value propogate from the static buffer allocation.  Any standards conforming
implementation of `snprintf` will null-terminate the output unless the buffer is
NULL (a zero-sized buffer is passed to the call).  On Windows, where this is not
the case, the function is named `_snprintf` which ensures that we do not
accidentally end up with the incorrect behaviour.

lib/Basic/Demangle.cpp

@@ -37,13 +37,13 @@ static void unreachable(const char *Message) {
 
 DemanglerPrinter &DemanglerPrinter::operator<<(unsigned long long n) & {
   char buffer[32];
-  snprintf(buffer, 32, "%llu", n);
+  snprintf(buffer, sizeof(buffer), "%llu", n);
   Stream.append(buffer);
   return *this;
 }
 DemanglerPrinter &DemanglerPrinter::operator<<(long long n) & {
   char buffer[32];
-  snprintf(buffer, 32, "%lld",n);
+  snprintf(buffer, sizeof(buffer), "%lld",n);
   Stream.append(buffer);
   return *this;
 }