As we do for IrFuncTyArg. We'll need access to the front-end linkage for
the Posix ABIs too in order to implement indirect-by-value passing of
non-PODs for `extern(C++)`.
Most importantly, make sure sret is enforced for all non-POD structs as
done for all other ABIs, and don't treat extern(D) any different from the
C ABI wrt. return values anymore.
Also reduce code duplication via common TargetABI patterns.
And for sret args too, except for MSVC targets.
At least on Win64, the sret align attributes lead to ICEs for std.bitmanip
and std.uni (release only), when computing the known alignment in the LLVM
inliner pass:
Assertion failed: I->getParent()->getParent() ==
Q.CxtI->getParent()->getParent() && "Got assumption for the wrong function!",
file C:\LDC\llvm\lib\Analysis\ValueTracking.cpp, line 711
0.15.1 used stdcall convention everywhere. But the ABI change in 0.15.2
requires use of the C calling convention for variadic calls on x86
because the stdcall convention does not support vararg functions.
Solution is to check the llvm::FunctionType if the function has varargs.
This commit also makes the C calling convention the global default
because according to the LLVM documentation the fastcc convention used
for D linkage does not support varargs.
This fixes issue #1000.
