TableGen: use fully qualified name for StringLiteral

Use the qualified name for StringLiteral (llvm::StringLiteral) when
generating the sources.  This is needed as the generated files may be
used out-of-tree (e.g. swift) where you may not have a
`using namespace llvm;` resulting in an undefined lookup.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293577 91177308-0d34-0410-b5e6-96231b3b80d8