[NVPTX] Handle addrspacecast constant expressions in aggregate initializers

We need to track if an AddrSpaceCast expression was seen when
generating an MCExpr for a ConstantExpr.  This change introduces a
custom lowerConstant method to the NVPTX asm printer that will create
NVPTXGenericMCSymbolRefExpr nodes at the appropriate places to encode
the information that a given symbol needs to be casted to a generic
address.

llvm-svn: 236000