Add mangling for externally inlineable specializations

An upcoming change has the SIL Optimizer drop the [fragile]
attribute from the specialized callee, unless the caller
is itself [fragile].

Since we need to distinguish specializations from fragile
and non-fragile contexts, add a new mangling node to
represent this concept.

lib/Basic/Remangle.cpp

@@ -485,6 +485,10 @@ void Remangler::mangleSpecializationPassID(Node *node) {
   Out << node->getIndex();
 }
 
+void Remangler::mangleSpecializationIsFragile(Node *node) {
+  Out << "q";
+}
+
 void Remangler::mangleFunctionSignatureSpecializationParam(Node *node) {
   if (!node->hasChildren()) {
     Out << "n_";