fix some issues with --backend (#14363)

* fix some issues with --backend
* fix https://github.com/timotheecour/Nim/issues/175; improve upon #14306

tests/misc/mbackend.nim

@@ -0,0 +1,31 @@
+#[
+We can't merge this test inside a `when defined(cpp)` because some bug that was
+fixed would not trigger in that case.
+]#
+
+import std/compilesettings
+import std/unittest
+
+static:
+  ## bugfix 1: this used to CT error with: Error: unhandled exception: mimportcpp.nim(6, 18) `defined(cpp)`
+  doAssert defined(cpp)
+  doAssert querySetting(backend) == "cpp"
+
+  ## checks that `--backend:c` has no side effect (ie, can be overridden by subsequent commands)
+  doAssert not defined(c)
+  doAssert not defined(js)
+  doAssert not defined(js)
+
+type
+  std_exception {.importcpp: "std::exception", header: "<exception>".} = object
+proc what(s: std_exception): cstring {.importcpp: "((char *)#.what())".}
+
+var isThrown = false
+try:
+  ## bugfix 2: this used to CT error with: Error: only a 'ref object' can be raised
+  raise std_exception()
+except std_exception as ex:
+  doAssert ex.what().len > 0
+  isThrown = true
+
+doAssert isThrown