Fixes SR-14951 : Print message of @available attribute within Protocol

Author: RAJAGOPALAN-GANGADHARAN

include/swift/AST/DiagnosticsSema.def

@@ -2311,8 +2311,8 @@ NOTE(declared_protocol_conformance_here,none,
      (Type, unsigned, Identifier, Identifier))
 
 ERROR(witness_unavailable,none,
-        "unavailable %0 %1 was used to satisfy a requirement of protocol %2",
-        (DescriptiveDeclKind, DeclName, Identifier))
+        "unavailable %0 %1 was used to satisfy a requirement of protocol %2%select{|: %3}3",
+        (DescriptiveDeclKind, DeclName, Identifier, StringRef))
 
 ERROR(redundant_conformance,none,
       "redundant conformance of %0 to protocol %1", (Type, Identifier))

lib/Sema/TypeCheckProtocol.cpp

@@ -4198,11 +4198,12 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
                                     NormalProtocolConformance *conformance) {
           auto &diags = witness->getASTContext().Diags;
           SourceLoc diagLoc = getLocForDiagnosingWitness(conformance, witness);
-          diags.diagnose(diagLoc,
-                         diag::witness_unavailable,
-                         witness->getDescriptiveKind(),
-                         witness->getName(),
-                         conformance->getProtocol()->getName());
+          auto *attr = AvailableAttr::isUnavailable(witness);
+          EncodedDiagnosticMessage EncodedMessage(attr->Message);
+          diags.diagnose(diagLoc, diag::witness_unavailable,
+                         witness->getDescriptiveKind(), witness->getName(),
+                         conformance->getProtocol()->getName(),
+                         EncodedMessage.Message);
           emitDeclaredHereIfNeeded(diags, diagLoc, witness);
           diags.diagnose(requirement, diag::kind_declname_declared_here,
                          DescriptiveDeclKind::Requirement,