[clang] fix crash on template instantiation of invalid requires expressions

See PR48656.

The implementation of the template instantiation of requires expressions
was incorrectly trying to get the expression from an 'ExprRequirement'
before checking if it was an error state.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D107399