Some additions to constant initializer generation code. Adds

bit-field initialization; ugly code, X86-only, but it works, at least 
for basic stuff.  Separates/adds union initialization; currently disabled,
though, because the struct/array code needs modifications to support 
elements of the wrong type.

Fixes PR2381 and PR2309 with the bit-field initialization.  And NetHack 
compiles and appears to work with a few tweaks (to work around the lack 
of transparent_union support, and clang being a bit strict about
conflicting declarations).

llvm-svn: 51763