syntax: better error when using "function" in POSIX mode

When parsing in POSIX mode, we already gave a good error message
when a user used the "function foo()" Bash syntax:

    $ shfmt -p <<<'function foo() { bar; }'
    <standard input>:1:13: the "function" builtin exists in bash; tried parsing as posix

However, our heuristic depended on the opening parenthesis token,
so it didn't work without the parentheses, which is valid Bash:

    $ shfmt -p <<<'function foo { bar; }'
    <standard input>:1:21: "}" can only be used to close a block

This "can only be used to close a block" error was rather confusing,
and caused multiple users to file bugs thinking the parser was at fault.
We now catch this other common pitfall pattern as well:

    $ shfmt -p <<<'function foo { bar; }'
    <standard input>:1:14: the "function" builtin is a bash feature; tried parsing as posix

We add the logic to the bit of code that handles each call argument,
but there shouldn't be a noticeable impact to most users
since it only kicks in when we're parsing in POSIX mode
and the current argument is a simple "{" literal.

Fixes #993.

syntax/parser.go

@@ -2422,6 +2422,10 @@ loop:
  ce.Assigns = append(ce.Assigns, p.getAssign(true))
  break
  }
+ // Avoid failing later with the confusing "} can only be used to close a block".
+ if p.lang == LangPOSIX && p.val == "{" && w != nil && w.Lit() == "function" {
+ p.curErr("the %q builtin is a bash feature; tried parsing as posix", "function")
+ }
  ce.Args = append(ce.Args, p.wordOne(p.lit(p.pos, p.val)))
  p.next()
  case _Lit:
@@ -2453,7 +2457,7 @@ loop:
  // Note that we'll only keep the first error that happens.
  if len(ce.Args) > 0 {
  if cmd := ce.Args[0].Lit(); p.lang == LangPOSIX && isBashCompoundCommand(_LitWord, cmd) {
- p.curErr("the %q builtin exists in bash; tried parsing as posix", cmd)
+ p.curErr("the %q builtin is a bash feature; tried parsing as posix", cmd)
  }
  }
  p.curErr("a command can only contain words and redirects; encountered %s", p.tok)

syntax/parser_test.go

@@ -1856,15 +1856,19 @@ var shellTests = []errorCase{
  },
  {
  in: "function foo() { bar; }",
- posix: `1:13: the "function" builtin exists in bash; tried parsing as posix`,
+ posix: `1:13: the "function" builtin is a bash feature; tried parsing as posix`,
+ },
+ {
+ in: "function foo { bar; }",
+ posix: `1:14: the "function" builtin is a bash feature; tried parsing as posix`,
  },
  {
  in: "declare foo=(bar)",
- posix: `1:13: the "declare" builtin exists in bash; tried parsing as posix`,
+ posix: `1:13: the "declare" builtin is a bash feature; tried parsing as posix`,
  },
  {
  in: "let foo=(bar)",
- posix: `1:9: the "let" builtin exists in bash; tried parsing as posix`,
+ posix: `1:9: the "let" builtin is a bash feature; tried parsing as posix`,
  },
  {
  in: "echo <(",