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/trunner.nim

@@ -0,0 +1,46 @@
+discard """
+  joinable: false
+"""
+
+## tests that don't quite fit the mold and are easier to handle via `execCmdEx`
+## A few others could be added to here to simplify code.
+
+import std/[strformat,os,osproc,strutils]
+
+proc runCmd(file, options = ""): auto =
+  let mode = if existsEnv("NIM_COMPILE_TO_CPP"): "cpp" else: "c"
+  const nim = getCurrentCompilerExe()
+  const testsDir = currentSourcePath().parentDir
+  let fileabs = testsDir / file.unixToNativePath
+  doAssert fileabs.existsFile, fileabs
+  let cmd = fmt"{nim} {mode} {options} --hints:off {fileabs}"
+  result = execCmdEx(cmd)
+  when false: # uncomment if you need to debug
+    echo result[0]
+    echo result[1]
+
+proc testCodegenStaticAssert() =
+  let (output, exitCode) = runCmd("ccgbugs/mstatic_assert.nim")
+  doAssert "sizeof(bool) == 2" in output
+  doAssert exitCode != 0
+
+proc testCTFFI() =
+  let (output, exitCode) = runCmd("vm/mevalffi.nim", "--experimental:compiletimeFFI")
+  let expected = """
+hello world stderr
+hi stderr
+foo
+foo:100
+foo:101
+foo:102:103
+foo:102:103:104
+foo:0.03:asdf:103:105
+ret={s1:foobar s2:foobar age:25 pi:3.14}
+"""
+  doAssert output == expected, output
+  doAssert exitCode == 0
+
+when defined(nimHasLibFFIEnabled):
+  testCTFFI()
+else: # don't run twice the same test
+  testCodegenStaticAssert()