Don't try to parse #if conditions as C expressions

Doing this required treating the newline that follows these conditions as significant
in order to avoid conflicts, but this caused problems for error recovery.

corpus/preprocessor.txt

@@ -107,9 +107,7 @@ General if blocks
 
 (translation_unit
   (preproc_if
-    (logical_expression
-      (call_expression (identifier) (argument_list (identifier)))
-      (call_expression (identifier) (argument_list (identifier))))
+    (preproc_arg)
     (preproc_def (identifier) (preproc_arg))))
 
 ============================================
@@ -131,7 +129,7 @@ int main() {
     (identifier)
     (function_declarator (identifier))
     (compound_statement
-      (preproc_if (identifier)
+      (preproc_if (preproc_arg)
         (return_statement (number_literal))
         (preproc_else
           (return_statement (number_literal)))))))

grammar.js

@@ -129,8 +129,7 @@ module.exports = grammar({
 
     preproc_if: $ => seq(
       '#if',
-      $._expression,
-      '\n',
+      $.preproc_arg,
       repeat($._top_level_item),
       optional($.preproc_else),
       '#endif'

src/grammar.json

@@ -268,11 +268,7 @@
         },
         {
           "type": "SYMBOL",
-          "name": "_expression"
-        },
-        {
-          "type": "STRING",
-          "value": "\n"
+          "name": "preproc_arg"
         },
         {
           "type": "REPEAT",