cgo: implement the constant parser as a real parser

Previously it was just a combination of heuristics to try to fit a
constant in an *ast.BasicLit. For more complex expressions, this is not
enough.

This change also introduces proper syntax error with locations, if
parsing a constant failed. For example, this will print a real error
message with source location:

    #define FOO 5)

cgo/cgo.go

@@ -42,7 +42,7 @@ type cgoPackage struct {
 // constantInfo stores some information about a CGo constant found by libclang
 // and declared in the Go AST.
 type constantInfo struct {
-	expr *ast.BasicLit
+	expr ast.Expr
 	pos  token.Pos
 }