[NFC] Mark SourceManager::getIDForBufferIdentifier const

include/swift/Basic/SourceManager.h

@@ -169,7 +169,7 @@ public:
 
   /// Returns a buffer ID for a previously added buffer with the given
   /// buffer identifier, or None if there is no such buffer.
-  Optional<unsigned> getIDForBufferIdentifier(StringRef BufIdentifier);
+  Optional<unsigned> getIDForBufferIdentifier(StringRef BufIdentifier) const;
 
   /// Returns the identifier for the buffer with the given ID.
   ///

lib/Basic/SourceLoc.cpp

@@ -152,9 +152,8 @@ SourceManager::getVirtualFile(SourceLoc Loc) const {
   return nullptr;
 }
 
-
-Optional<unsigned> SourceManager::getIDForBufferIdentifier(
-    StringRef BufIdentifier) {
+Optional<unsigned>
+SourceManager::getIDForBufferIdentifier(StringRef BufIdentifier) const {
   auto It = BufIdentIDMap.find(BufIdentifier);
   if (It == BufIdentIDMap.end())
     return None;