[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 271e495399170d69627c1acd591c9298cb0b5b4b)