clang/test/CodeGenCXX/debug-info-static-member.cpp: Appease targetting msvc to add explicit -target x86_64-unknown-unknown.

with -target i686-win32, you will see;

debug-info-static-member.cpp:11:22: error: in-class initializer for static data member of type 'const float' requires 'constexpr' specifier [-Wstatic-float-init]
  const static float const_b = 3.14;
                     ^         ~~~~
  constexpr