[Support] Add const to `FileError::getFileName`

[bnbarham]

Summary

getFileName returns a StringRef, there's no reason it shouldn't be
const.

[bnbarham]

@dexonsmith added you to reviewers since you were automatically added as a subscriber :)

[dexonsmith]

LGTM.

llvm/include/llvm/Support/Error.h

public:
 
   std::string messageWithoutFileInfo() const {
     std::string Msg;
     raw_string_ostream OS(Msg);
     Err->log(OS);
     return OS.str();
   }
 
-  StringRef getFileName() { return FileName; }
+  StringRef getFileName() const { return FileName; }
 
   Error takeError() { return Error(std::move(Err)); }
 
   std::error_code convertToErrorCode() const override;
 
   // Used by ErrorInfo::classID.
   static char ID;