[C++17] Implement N3928 - Extending static_assert

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

This extends' static assert to not require a message string.
I elected to make this work also for C++11 and C++14 and warn only with 
-pedantic.
I think many people just write
  static_assert(thing, "");
.

I took the path of building an empty string in the parser in this case.
I wasn't sure if setting message to NULL_TREE would cause sadness later 
on or not.

I also, perhaps in a fit of overzealousness made finish_static_assert 
not print the extra ": " and an empty message in this case.

I didn't modify _Static_assert for C.

Built and tested on x86_64-linux.

OK

Attachment: CL_static_assert_nomsg
Description: Text document

Attachment: patch_static_assert_nomsg
Description: Text document