Add ImportC compiler to dmd

[Walter Bright]

On 5/10/2021 4:01 PM, deadalnix wrote:
> inline change the linker flags so that multiple definitions do not fail. That's 
> all it does.

They're still done with COMDATs (the equivalent in Elf, Mach, MSCoff).

A COMDAT is a separate section. One COMDAT per function means they can be 
handled separately. The pre-1990 way was to lump everything into one section, 
and the different functions could not be teased out of that.

Some of the more peculiar C linkage semantics are sad attempts to deal with no 
COMDAT support.