Fix invalid TestBitfields case (thanks Filipe!), and xfail one case due to GCC compil...

Fix invalid TestBitfields case (thanks Filipe!), and xfail one case due to GCC compiler bug.

Turns out unnamed bit fields should not be accessed ever in C (C99 section 4.7.8) or C++
(C++11 section 9.6).

Unrelated to the above, this commit marks TestBitfields as expected-to-fail with GCC as 4.7
has problems with padding... the size of the struct with a uint32_t member is reported as 12
when GCC is used. Clang emits correct code.

Parts of this patch prepared by Filipe Cabecinhas!

llvm-svn: 175666