Avoid a Symbol -> Name -> Symbol conversion.

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