Add support for static_assert

Add C11's _Static_assert and static_assert macro.

This is like #error, but is handled at a later stage
of translation, so it is possible to check sizes of
types, values of enums, etc.

https://en.cppreference.com/w/c/language/_Static_assert
https://port70.net/~nsz/c/c11/n1570.html#6.7.10

test/err/staticassert.c

@@ -0,0 +1,26 @@
+/*
+  Copyright 2020 Google LLC
+
+  This software is provided 'as-is', without any express or implied
+  warranty. In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/*
+** Test of failing _Static_assert.
+**/
+
+
+_Static_assert(0, "0 should be false.");