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.

Author: compnerd

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;
 }

lib/Sema/PlaygroundTransform.cpp

@@ -889,7 +889,7 @@ class Instrumenter {
   std::pair<PatternBindingDecl*, VarDecl*>
     buildPatternAndVariable(Expr *InitExpr) {
     char NameBuf[11] = { 0 };
-    snprintf(NameBuf, 11, "tmp%u", TmpNameIndex);
+    snprintf(NameBuf, sizeof(NameBuf), "tmp%u", TmpNameIndex);
     TmpNameIndex++;
 
     Expr *MaybeLoadInitExpr = nullptr;

stdlib/public/runtime/Reflection.mm

@@ -437,8 +437,7 @@ void swift_TupleMirror_subscript(String *outString,
 
   // The name is the stringized element number '.0'.
   char buf[32];
-  snprintf(buf, 31, ".%zd", i);
-  buf[31] = 0;
+  snprintf(buf, sizeof(buf), ".%zd", i);
   new (outString) String(buf, strlen(buf));
 
   // Get a Mirror for the nth element.