incomplete importc types should be annotated with incomplete and optional sizeof / alignof pragmas

[timotheecour]

/cc @krux02
this would help with #5664 (implement sizeof and alignof operator) for importc types:

cda1ab4#diff-3ff4bd5c972c5629ed4e7e62596e3b3bR326
# TODO there is no proper way to find out if a type cannot be queried for the size.

as well as here:

cda1ab4#diff-e3317694adf0c30fc5d1562cb85f4d40R2

discard """
errormsg: "cannot evaluate 'sizeof/alignof' because its type is not defined completely"
line: 9
"""
 type
  MyStruct {.importc: "MyStruct".} = object
 const i = sizeof(MyStruct)
 echo i

instead of cannot evaluate 'sizeof/alignof' because its type is not defined completely for all importc types we can restrict this error to cases where type is marked with {.incomplete.}

Furthermore, we can add optional annotations {.sizeof: 32, alignof:32.} pragma for cases of {.incomplete.} types T so that T.sizeof can just use that directly, when available

[mratsim]

There is a size pragma for imported types, though it is very restricted at the moment: #7674

[krux02]

That is isteresting. But for a recursive calculation of object sizes it is important to know borth size and alignment. Just setting the size does not really help.

[krux02]

Alignment is often very platform, compiler and compiler flag dependent. you can't just measure alignment and size and set it as a constant. If something goes wrong here, debugging will be horrible.