[Clang][Lex] Fix parsing of nested requirement to prevent flowing off the end of token stream

Currently when parsing a nested requirement we attempt to balance parens if we
have a parameter list. This will fail in some cases of ill-formed code and keep
going until we fall off the token stream and crash. This fixes the hand parsing
by using SkipUntil which will properly flag if we don't find the expected
tokens.

Fixes: #73112

Author: shafik

clang/docs/ReleaseNotes.rst

@@ -783,6 +783,9 @@ Bug Fixes to C++ Support
   completes (except deduction guides). Fixes:
   (`#59827 <https://github.com/llvm/llvm-project/issues/59827>`_)
 
+- Fix crash when parsing nested requirement. Fixes:
+  (`#73112 <https://github.com/llvm/llvm-project/issues/73112>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/Parse/ParseExprCXX.cpp

@@ -3635,10 +3635,12 @@ ExprResult Parser::ParseRequiresExpression() {
               auto Res = TryParseParameterDeclarationClause();
               if (Res != TPResult::False) {
                 // Skip to the closing parenthesis
-                // FIXME: Don't traverse these tokens twice (here and in
-                //  TryParseParameterDeclarationClause).
                 unsigned Depth = 1;
                 while (Depth != 0) {
+                  bool FoundParen = SkipUntil(tok::l_paren, tok::r_paren,
+                                              SkipUntilFlags::StopBeforeMatch);
+                  if (!FoundParen)
+                    break;
                   if (Tok.is(tok::l_paren))
                     Depth++;
                   else if (Tok.is(tok::r_paren))

clang/test/Parser/cxx2a-concepts-requires-expr.cpp

@@ -160,3 +160,11 @@ template <int N>
 requires requires {
  typename BitInt<N>; // ok
 } using r44 = void;
+
+namespace GH73112 {
+void f() {
+    requires { requires(int; } // expected-error {{expected ')'}} \
+                               // expected-error {{expected expression}} \
+                               // expected-note {{to match this '('}}
+}
+}