DIP45: fixing the dllimport/dllexport issue

[Benjamin Thaut]

So let me summarize the currently discussed solutions:

1) Use alias symbols
Pros:
  - Better usability (no additional command line parameters needed when 
compiling / linking against DLLs)

Cons:
  - Less efficient code for cross DLL function calls (one additional 
call instruction)
  - Less efficient code for all accesses to global data, no matter if it 
will end up in a DLL or not. That means __gshared variables, global 
shared variables, hidden global data like module info, type info, 
vtables, etc (one additional level of indirection). It might be possible 
to avoid this in most cases using link time optimization. But it is 
unlikely that we can get LTO to work easily.
  - Additional level of indirection might confuse current debuggers

2) Use additional command line
Pros:
  - More efficient code

Cons:
  - Additional command line parameters needed when compiling / linking 
against a DLL. (can be hidden away inside sc.ini for phobos / druntime)