Finish building the full-expression for a static_assert expression

before evaluating it rather than afterwards.

This is groundwork for C++20's P0784R7, where non-trivial destructors
can be constexpr, so we need ExprWithCleanups markers in constant
expressions.

No significant functionality change intended (though this fixes a bug
only visible through libclang / -ast-dump / tooling: we now store the
converted condition on the StaticAssertDecl rather than the original).

llvm-svn: 372368