Introduce StringRefZ class to represent null-terminated strings.

StringRefZ is a class to represent a null-terminated string. String
length is computed lazily, so it's more efficient than StringRef to
represent strings in string table.

The motivation of defining this new class is to merge functions
that only differ in string types; we have many constructors that takes
`const char *` or `StringRef`. With StringRefZ, we can merge them.

Differential Revision: https://reviews.llvm.org/D27037

llvm-svn: 288172