Use a better name for compile unit labels.

They mark the start of a compile unit, so name them .Lcu_*. Using
Section->getLabelBeginName() makes it looks like they mark the start of the
section.

While at it, switch to createTempSymbol to avoid collisions with labels
created in inline assembly. Not sure if a "don't crash" test is worth it.

With this getLabelBeginName is dead, delete it.

llvm-svn: 231750