[Concepts] Add missing TPA commit to requires expression parsing

If an error had occurred when annotating a scope spec during the tentative parse
for a type-requirement, we would not revert nor commit the tentative parse, triggerring
an assertion failure.

Commit the TPA in this case and then do error recovery.

(cherry picked from commit 271e495)

Author: saarraz

clang/lib/Parse/ParseExprCXX.cpp

@@ -3476,6 +3476,7 @@ ExprResult Parser::ParseRequiresExpression() {
           // We need to consume the typename to allow 'requires { typename a; }'
           SourceLocation TypenameKWLoc = ConsumeToken();
           if (TryAnnotateCXXScopeToken()) {
+            TPA.Commit();
             SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
             break;
           }