This reverts commit c117c2c8ba.
itaniumDemangle calls std::strlen with the results of
std::string_view::data() which may not be NUL-terminated. This causes
lld/test/wasm/why-extract.s to fail when "expensive checks" are enabled
via -DLLVM_ENABLE_EXPENSIVE_CHECKS=ON. See D149675 for further
discussion. Back this out until the individual demanglers are converted
to use std::string_view.