refactor(3p/nix/libexpr): Extract static helpers out of parser.y

Moves several of the static helper functions into a new parser.cc
file.

Once the rest of the code is usefully extracted, these will be moved
to a private namespace.

Change-Id: I0d7b53dcefe31bb5c6bad3ad7f5fcb48276bf799
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1218
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/parser.hh

@@ -15,6 +15,8 @@
   int yylex(YYSTYPE* yylval_param, YYLTYPE* yylloc_param, yyscan_t yyscanner, \
             nix::ParseData* data)
 
+#define CUR_POS makeCurPos(*yylocp, data)
+
 namespace nix {
 
 struct ParseData {
@@ -44,4 +46,12 @@ static void dupAttr(Symbol attr, const Pos& pos, const Pos& prevPos) {
                    attr % pos % prevPos);
 }
 
+void addAttr(ExprAttrs* attrs, AttrPath& attrPath, Expr* e, const Pos& pos);
+
+void addFormal(const Pos& pos, Formals* formals, const Formal& formal);
+
+Expr* stripIndentation(const Pos& pos, SymbolTable& symbols,
+                       std::vector<Expr*>& es);
+
+
 }  // namespace nix