[libcxxabi] copy back std::string_view patches from LLVM

[libcxxabi] copy back std::string_view patches from LLVM

I made a series of changes to LLVM's demangle in:
- D148348
- D148353
- D148363
- D148375
and so did Fangrui in 3ece37b3fa2c and Ashay in D149061.

I didn't notice the banner about there being two copies of this in tree
and was modifying the downstream versions. Copy these changes back to
the upstream version. Oops!

Reviewed By: MaskRay, #libc_abi, ldionne, phosek

Differential Revision: https://reviews.llvm.org/D148566