[LLDB] Fix assertion failure by removing `CopyType` in `std::coroutine_handle` pretty printer

The pretty printer for `std::coroutine_handle` was running into
> Assertion failed: (target_ctx != source_ctx && "Can't import into itself")
from ClangASTImporter.h, line 270.

This commit fixes the issue by removing the `CopyType` call from the
pretty printer. While this call was necessary in the past, it seems to
be no longer required, at least all test cases are still passing. Maybe
something changed in the meantime around the handling of `TypesystemClang`
instances. I don't quite understand why `CopyType` was necessary earlier.

I am not sure how to add a regression test for this, though. It seems
the issue is already triggered by the exising `TestCoroutineHandle.py`,
but API tests seem to ignore all violations of `lldbassert` and still
report the test as "passed", even if assertions were triggered

Differential Revision: https://reviews.llvm.org/D143127