ConstantArray::get() (for strings) is going away, use

ConstantDataArray::getString instead.

Many instances of ConstantArray::get() could be moved to 
use more efficient ConstantDataArray methods that avoid a ton
of intermediate Constant*'s for each element (e.g. 
GetConstantArrayFromStringLiteral).  I don't plan on doing this
in the short-term though.

llvm-svn: 149363