[llvm] Fix string copy confusion

The microsoft demangler makes copies of the demangled strings, but has
some confusion between StringView representation (sans NUL), and
C-strings (with NUL).  Here we also have a use of strcpy, which
happens to work because the incoming string view happens to have a
trailing NUL.  But a simple memcpy excluding the NUL is sufficient.

Reviewed By: dblaikie, erichkeane

Differential Revision: https://reviews.llvm.org/D122391