[Demangle] avoid more std::string_view::substr

[Demangle] avoid more std::string_view::substr

In D148959, I removed usage of std::string_view::substr because it may
throw, and libcxxabi cannot use such code.  I missed one instance in
llvm::starts_with.  That is blocking copying the code back upstream in
D148566.

Mark these helpers noexcept (as they are in C++20) as well, to remind
future travelers.

Make these changes upstream, and copy them back downstream using
libcxxabi/src/demangle/cp-to-llvm.sh.

Reviewed By: #libc_abi, MaskRay, ldionne

Differential Revision: https://reviews.llvm.org/D151260