Avoid a Symbol -> Name -> Symbol conversion.

Before this we were producing a TargetExternalSymbol from a MCSymbol.
That meant extracting the symbol name and fetching the symbol again
down the pipeline.

This patch adds a DAG.getMCSymbol that lets the MCSymbol pass unchanged on the
DAG.

Doing so removes the need for MO_NOPREFIX and fixes the root cause of pr23900,
allowing r240130 to be committed again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240300 91177308-0d34-0410-b5e6-96231b3b80d8

utils/TableGen/CodeGenDAGPatterns.cpp

@@ -2143,7 +2143,8 @@ TreePatternNode *TreePattern::ParseTreePattern(Init *TheInit, StringRef OpName){
         Operator->getName() != "tblockaddress" &&
         Operator->getName() != "tglobaladdr" &&
         Operator->getName() != "bb" &&
-        Operator->getName() != "vt")
+        Operator->getName() != "vt" &&
+        Operator->getName() != "mcsym")
       error("Cannot use '" + Operator->getName() + "' in an output pattern!");
   }