`demangle`: Add complexity guard to `ParseQRequiresExpr`

This function copies the parser's state onto the stack in order to perform backtracking.
Adding a complexity guard will ensure that it fails gracefully, instead of contributing to excessive consumption of stack space.

PiperOrigin-RevId: 606720511
Change-Id: I2f6f03e5f8bc4cc571a4159ecfc2af6f3e00fa68

absl/debugging/internal/demangle.cc

@@ -1942,6 +1942,8 @@ static bool ParseExprCastValue(State *state) {
 // template <typename T>
 // int foo(T);
 static bool ParseQRequiresClauseExpr(State *state) {
+  ComplexityGuard guard(state);
+  if (guard.IsTooComplex()) return false;
   ParseState copy = state->parse_state;
   DisableAppend(state);