fixes #18896; fixes #20886; importc types alias doesn't work with distinct

[ringabout]

fixes #18896
fixes #20886

type
  PFile {.importc: "FILE*", header: "<stdio.h>".} = distinct pointer
    # import C's FILE* type; Nim will treat it as a new pointer type

This is an excerpt from the Nim manual. In the old Nim versions, it produces a void pointer type instead of the FILE* type that should have been generated. Because these C types tend to be opaque and adapt to changes on different platforms. It might affect the portability of Nim on various OS, i.e. csource_v2 cannot build on the apline platform because of Time relies on Nim types instead of the time_t type.

ref #18851

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The lines below are statistics of the Nim compiler built from 8be82c3

Hint: mm: orc; opt: speed; options: -d:release
175264 lines; 8.308s; 655.316MiB peakmem