[c++20] Implement tweaked __VA_OPT__ rules from P1042R1:

[c++20] Implement tweaked __VA_OPT__ rules from P1042R1:

 * __VA_OPT__ is expanded if the *expanded* __VA_ARGS__ is non-empty,
   not if the original argument contained no tokens.
 * Placemarkers at the start and end of __VA_OPT__ are retained just
   long enough to paste them with adjacent ## operators. We never paste
   "across" a discarded placemarker.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359964 91177308-0d34-0410-b5e6-96231b3b80d8