[Remarks] Add support for prepending a path to external files

This helps with testing and debugging for paths that are assumed
absolute.

It also uses a FileError to provide the file path it's trying to open.

llvm-svn: 375008
diff --git a/llvm/lib/Remarks/BitstreamRemarkParser.h b/llvm/lib/Remarks/BitstreamRemarkParser.h
index f1f28b4..7c9cc2f 100644
--- a/llvm/lib/Remarks/BitstreamRemarkParser.h
+++ b/llvm/lib/Remarks/BitstreamRemarkParser.h
@@ -73,9 +73,9 @@
   Error processExternalFilePath(Optional<StringRef> ExternalFilePath);
 };
 
-Expected<std::unique_ptr<BitstreamRemarkParser>>
-createBitstreamParserFromMeta(StringRef Buf,
-                              Optional<ParsedStringTable> StrTab = None);
+Expected<std::unique_ptr<BitstreamRemarkParser>> createBitstreamParserFromMeta(
+    StringRef Buf, Optional<ParsedStringTable> StrTab = None,
+    Optional<StringRef> ExternalFilePrependPath = None);
 
 } // end namespace remarks
 } // end namespace llvm