[NFC] Replace StringMap<char, A> with StringSet<A> (swiftlang#27986)

Author: varungandhi-apple

include/swift/AST/DiagnosticEngine.h

@@ -21,7 +21,7 @@
 #include "swift/AST/TypeLoc.h"
 #include "swift/AST/DeclNameLoc.h"
 #include "swift/AST/DiagnosticConsumer.h"
-#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/VersionTuple.h"
 
@@ -655,7 +655,7 @@ namespace swift {
     /// A set of all strings involved in current transactional chain.
     /// This is required because diagnostics are not directly emitted
     /// but rather stored until all transactions complete.
-    llvm::StringMap<char, llvm::BumpPtrAllocator &> TransactionStrings;
+    llvm::StringSet<llvm::BumpPtrAllocator &> TransactionStrings;
 
     /// The number of open diagnostic transactions. Diagnostics are only
     /// emitted once all transactions have closed.

lib/AST/DiagnosticEngine.cpp

@@ -1026,7 +1026,7 @@ void DiagnosticEngine::onTentativeDiagnosticFlush(Diagnostic &diagnostic) {
     if (content.empty())
       continue;
 
-    auto I = TransactionStrings.insert(std::make_pair(content, char())).first;
+    auto I = TransactionStrings.insert(content).first;
     argument = DiagnosticArgument(StringRef(I->getKeyData()));
   }
 }