make clang include a FILE symbol for .(s|S) files

This is mostly needed by syspatch at the moment to be
to be able to re-link in the same order as the original
libraries were linked with by relying on the readelf(1)
and without this .(s|S) assembly files were not getting
a file directive.

Index: lib/MC/MCParser/AsmParser.cpp
--- lib/MC/MCParser/AsmParser.cpp.orig
+++ lib/MC/MCParser/AsmParser.cpp
@@ -975,6 +975,10 @@ bool AsmParser::Run(bool NoInitialTextSection, bool No
     (void)InsertResult;
   }
 
+  StringRef Filename = getContext().getMainFileName();
+  if (!Filename.empty() && (Filename.compare(StringRef("-")) != 0))
+    Out.emitFileDirective(Filename);
+
   getTargetParser().onBeginOfFile();
 
   // While we have input, parse each statement.