[PATCH] D40530: Factor out common code to Common/Strings.cpp.

[Rafael Avila de Espindola via llvm-commits]

Moving the itanium demangler to Common makes sense since it is used by
various formats and can be built everywhere.

I am not sure of the value of moving the msvc demangler. It is only used
by COFF and can only be built on windows.

Cheers,
Rafael