[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

llvm/lib/Demangle/MicrosoftDemangle.cpp

@@ -245,8 +245,8 @@ demanglePointerCVQualifiers(StringView &MangledName) {
 }
 
 StringView Demangler::copyString(StringView Borrowed) {
-  char *Stable = Arena.allocUnalignedBuffer(Borrowed.size() + 1);
-  std::strcpy(Stable, Borrowed.begin());
+  char *Stable = Arena.allocUnalignedBuffer(Borrowed.size());
+  std::memcpy(Stable, Borrowed.begin(), Borrowed.size());
 
   return {Stable, Borrowed.size()};
 }