Fix bug so that programIndex (et al.) are available in the debugger.

It's now possible to successfully print out the value of programIndex,
programCount, etc., in the debugger.  The issue was that they were
defined as having InternalLinkage, which meant that DCE removed them
at the end of compilation.  Now they're declared to have WeakODRLinkage,
which ensures that one copy survives (but there aren't multiply-defined
symbols when compiling multiple files.)

func.cpp

@@ -198,7 +198,7 @@ Function::emitCode(FunctionEmitContext *ctx, llvm::Function *function,
     // value
     maskSymbol->storagePtr = ctx->GetFullMaskPointer();
 
-    // add debugging info for __mask, programIndex, ...
+    // add debugging info for __mask
     maskSymbol->pos = firstStmtPos;
     ctx->EmitVariableDebugInfo(maskSymbol);