[C++20] [Modules] Remove assertion of current module when acting on import

Closes https://github.com/llvm/llvm-project/issues/58199

Previously, when we act on a import statement, we'll assume there is a
module declaration in the current TU if the command line tells us we're
compiling a module unit. This makes since on valid codes. However, for
invalid codes, it is possible. See
https://github.com/llvm/llvm-project/issues/58199 for example.

This patch removes the assertion. And the assertion is a noop and it
should be safe to remove it.