[ARM/AArch64] Enforce alignment for bitfielded structs

[ARM/AArch64] Enforce alignment for bitfielded structs

When creating a global variable with a type of a struct with bitfields, we must
forcibly set the alignment of the global from the RecordDecl. We must do this so
that the proper bitfield alignment makes its way down to LLVM, since clang will
mangle the bitfields into one large type.

llvm-svn: 235976