[Sema] Relax a failing assertion in TransformBlockExpr

The assertion fails when the expression causing the this pointer to be
captured is part of a constexpr if statement's branch and the branch
gets discarded when the enclosing method is instantiated.

Note that the test case is added to CodeGen instead of Sema since the
translation unit has to be free of errors in order for the assertion to
be checked.

Differential Revision: https://reviews.llvm.org/D144016