Pretty: do not wrap non-compound if/while/for bodies in braces

[jkoppel]

What

Language.C.Pretty synthesizes a CCompound around any non-compound
if/while/for body before printing it. This commit removes that
synthesis and prints the body as-is.

Example

parseTest :: String -> IO ()
parseTest src = case parseC (inputStreamFromString src) (initPos \"<demo>\") of
  Right tu -> putStrLn (render (pretty tu))
  Left  e  -> print e

> parseTest \"int f(int x) { if (x) return 1; return 0; }\"
int f(int x)
{
    if (x)
    {
        return 1;
    }
    return 0;
}

The output has braces around return 1; that weren't in the source — those braces are introduced entirely by the pretty-printer. With this fix, the second-from-bottom line is just if (x) return 1;.

Why

parse >> pretty doesn't round-trip on any if/while/for/else
whose body is a single non-compound statement, which is a very common
shape in real-world C. The current behavior at
Pretty.hs:128 is:

prettyBody nonCompound = prettyPrec (-1) (CCompound [] [CBlockStmt nonCompound] undefined)

It wraps the body in a fresh CCompound (with undefined as its
NodeInfo, which is also a footgun if any downstream consumer
later inspects the synthesized node).

The fix:

prettyBody nonCompound = pretty nonCompound

Brace wrapping stays in the hands of whoever built the AST — the
parser already wraps with CCompound whenever the source did, so
compound bodies still print with braces.

Credit

Originally written by @dvekeman in 2016 against the downstream
jkoppel/language-c fork
(issue #1 there); never submitted upstream. The commit is preserved
intact with its original author.

Tested

Cherry-picked onto current upstream master (00db59a); cubix's
parametric-syntax test suite (~100 tests including round-trip checks
on a real C corpus) passes against the resulting language-c. Happy
to add a targeted test inside language-c itself if helpful.