Avoid keeping internal string_views in Twine.

This is a follow-up to https://reviews.llvm.org/D103935

A Twine's internal layout should not depend on which version of the
C++ standard is in use. Dynamically linking binaries compiled with two
different layouts (eg, --std=c++14 vs --std=c++17) ends up
problematic.

This change avoids that issue by immediately converting a
string_view to a pointer-and-length at the cost of an extra eight-bytes
in Twine.

Differential Revision: https://reviews.llvm.org/D106186