[MCJIT] Teach RuntimeDyldChecker to handle underscores at the start of symbols.

RuntimeDyldChecker had been testing isalpha(Expr[0]) to recognise symbol tokens,
and throwing unrecognized token errors when it hit symbols with leading
underscores. This fixes that.

llvm-svn: 213706
diff --git a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldChecker.cpp b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldChecker.cpp
index 72075f5..308d80ef 100644
--- a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldChecker.cpp
+++ b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldChecker.cpp
@@ -519,7 +519,7 @@
       std::tie(SubExprResult, RemainingExpr) = evalParensExpr(Expr, PCtx);
     else if (Expr[0] == '*')
       std::tie(SubExprResult, RemainingExpr) = evalLoadExpr(Expr);
-    else if (isalpha(Expr[0]))
+    else if (isalpha(Expr[0]) || Expr[0] == '_')
       std::tie(SubExprResult, RemainingExpr) = evalIdentifierExpr(Expr, PCtx);
     else if (isdigit(Expr[0]))
       std::tie(SubExprResult, RemainingExpr) = evalNumberExpr(Expr);