[MetaRenamer] Leave @main alone.

To the best of my knowledge -metarenamer is used in two cases:
1) obfuscate names, when e.g. they contain informations that
can't be shared.
2) Improve clarity of the textual IR for testcases.

One of the usecases if getting the output of `opt` and passing it
to the lli interpreter to run the test. If metarenamer renames
@main, lli can't find an entry point.

llvm-svn: 309657

llvm/lib/Transforms/Utils/MetaRenamer.cpp

@@ -123,7 +123,11 @@ namespace {
             TLI.getLibFunc(F, Tmp))
           continue;
 
-        F.setName(renamer.newName());
+        // Leave @main alone. The output of -metarenamer might be passed to
+        // lli for execution and the latter needs a main entry point.
+        if (Name != "main")
+          F.setName(renamer.newName());
+
         runOnFunction(F);
       }
       return true;