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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149363 91177308-0d34-0410-b5e6-96231b3b80d8