[libc] Add a standalone flavor of an equivalent of std::string_view.

This class is to serve as a replacement for llvm::StringRef as part of
the plans to limit dependency on other parts of LLVM. One use of
llvm::StringRef in MPFRWrapper has been replaced with the new class.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D97330

GitOrigin-RevId: dbb131d53aacabdf5d85299e447a970a0280b127
6 files changed