Mark macros.error as .noreturn. (#23081)

Closes #14329 

Marks `macros.error` as `.noreturn` so that it can be used in
expressions. This also fixes the issue that occurred in #19659 where a
stmt that could be an expression (Due to having `discardable` procs at
the end of other branches) would believe a `noreturn` proc is returning
the same type e.g.
```nim
 proc bar(): int {.discardable.} = discard

if true: bar()
else: quit(0) # Says that quit is of type `int` and needs to be used/discarded except it actually has no return type
```

compiler/condsyms.nim

@@ -157,6 +157,7 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimAllowNonVarDestructor")
   defineSymbol("nimHasQuirky")
   defineSymbol("nimHasEnsureMove")
+  defineSymbol("nimHasNoReturnError")
 
   defineSymbol("nimUseStrictDefs")
   defineSymbol("nimHasNolineTooLong")

compiler/semstmts.nim

@@ -169,6 +169,11 @@ proc discardCheck(c: PContext, result: PNode, flags: TExprFlags) =
         while n.kind in skipForDiscardable:
           if n.kind == nkTryStmt: n = n[0]
           else: n = n.lastSon
+
+        # Ignore noreturn procs since they don't have a type
+        if n.endsInNoReturn:
+          return
+
         var s = "expression '" & $n & "' is of type '" &
             result.typ.typeToString & "' and has to be used (or discarded)"
         if result.info.line != n.info.line or

lib/core/macros.nim

@@ -427,7 +427,12 @@ proc copyNimTree*(n: NimNode): NimNode {.magic: "NCopyNimTree", noSideEffect.} =
       let x = 12
       echo x
 
-proc error*(msg: string, n: NimNode = nil) {.magic: "NError", benign.}
+when defined(nimHasNoReturnError):
+  {.pragma: errorNoReturn, noreturn.}
+else:
+  {.pragma: errorNoReturn.}
+
+proc error*(msg: string, n: NimNode = nil) {.magic: "NError", benign, errorNoReturn.}
   ## Writes an error message at compile time. The optional `n: NimNode`
   ## parameter is used as the source for file and line number information in
   ## the compilation error message.

tests/casestmt/tcasestmt.nim

@@ -298,3 +298,17 @@ proc main(a: uint64) =
 static:
   main(10)
 main(10)
+
+block:
+  # Just needs to compile
+  proc bar(): int {.discardable.} = discard
+
+  proc foo() {.noreturn.} = discard
+
+  case "*"
+  of "*":
+    bar()
+  else:
+    # Make sure this noreturn doesn't
+    # cause the discardable to not discard
+    foo()

tests/macros/t14329.nim

@@ -0,0 +1,4 @@
+import macros
+
+macro myMacro(n) =
+  let x = if true: newLit"test" else: error "error", n