[lldb] Change parameter type of StructuredData::ParseJSON

Instead of taking a `const std::string &` we can take an
`llvm::StringRef`. The motivation for this change is that many of the
callers of `ParseJSON` end up creating a temporary `std::string` from an existing
`StringRef` or `const char *` in order to satisfy the API. There's no
reason we need to do this.

Differential Revision: https://reviews.llvm.org/D148579