[ELF] - Enable testcases that uses demangler to run under windows and escape quotes around "C++".

Previously testcases were enabled only for shell.
r280733 added c++ itanium demangler to lld and we can enable them fully.

Also this change make quotes to be escaped:

extern "C++" -> extern \"C++\", which worked before just because we are dropping tokens 
quotes internally at this moment.

llvm-svn: 280797