Add ImportC compiler to dmd

[Walter Bright]

On 5/9/2021 4:27 PM, Max Haughton wrote:
> `inline` in the C standard (although given as an example C11 footnote 138) 
> technically *only* has guaranteed implications other than actually inlining the 
> function, is ignoring it valid?

C11 6.7.4-6 says:

"A function declared with an inline function specifier is an inline function. 
Making a function an inline function suggests that calls to the function be as 
fast as possible. The extent to which such suggestions are effective is 
implementation-defined."

Ignoring it is not only valid, it's what modern C compilers do anyway.