Bad codegen with {.exportc.} variant member

[yglukhov]

type Foo* = object
    case smth: bool
    of true:
        refval {.exportc.}: int
    else: nil

var v: Foo
v.smth = true
v.refval = 5 # Bad codegen!

Hint: system [Processing]
Hint: test [Processing]
CC: test
CC: stdlib_system
Error: execution of an external compiler program 'clang -c  -w  -I/Users/yglukhov/Projects/nim/lib -o /Users/yglukhov/nimcache/test.o /Users/yglukhov/nimcache/test.c' failed with exit code: 256

/Users/yglukhov/nimcache/test.c:149:21: error: no member named 'refval' in 'struct Foo95004'
        v_95007_2076107616.refval = ((NI) 5);
        ~~~~~~~~~~~~~~~~~~ ^
/Users/yglukhov/nimcache/test.c:166:20: error: no member named 'refval' in 'Foo95004'
TMP139[1].offset = offsetof(Foo95004, refval);
                   ^                  ~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/7.3.0/include/stddef.h:120:24: note: expanded from macro 'offsetof'
#define offsetof(t, d) __builtin_offsetof(t, d)
                       ^                     ~
2 errors generated.

[Araq]

I don't think we can support this, it's wrapped in a union.