[Support] Fix warnings

This patch fixes:

  llvm/unittests/Support/ScopedPrinterTest.cpp:519:20: error: unused
  variable 'InfDouble' [-Werror,-Wunused-variable]

  llvm/unittests/Support/ScopedPrinterTest.cpp:520:16: error: unused
  variable 'NaNDouble' [-Werror,-Wunused-variable]

  llvm/unittests/Support/ScopedPrinterTest.cpp:516:15: error: unused
  variable 'NaNFloat' [-Werror,-Wunused-variable]

  llvm/unittests/Support/ScopedPrinterTest.cpp:515:19: error: unused
  variable 'InfFloat' [-Werror,-Wunused-variable]

Since commit fa56e362 has temporarily
disabled tests involving these constants, this patch simply comments
them out instead of removing them.