fix some codegen bugs: NIM_BOOL, NIM_STATIC_ASSERT, --passc:-std=... (etc) (#13798)

* fix cgen bool D20200328T203812
* --passc:std=c++17 (etc) now works instead of silently ignored
* document caveats for NIM_NIL

tests/ccgbugs/tcodegenbug_bool.nim

@@ -0,0 +1,11 @@
+discard """
+"""
+
+# issue #13798
+{.emit:"""
+#include <stdbool.h>
+void fun(bool a){}
+""".}
+
+proc fun(a: bool) {.importc.}
+fun(true)