[CodeGen] Fix an assertion failure in CGRecordLowering.

This patch fixes a bug in CGRecordLowering::accumulateBitFields where it
unconditionally starts a new run and emits a storage field when it sees
a zero-sized bitfield, which causes an assertion in insertPadding to
fail when -fno-bitfield-type-align is used.

It shouldn't emit new storage if UseZeroLengthBitfieldAlignment and
UseBitFieldTypeAlignment are both false.

rdar://problem/36762205

llvm-svn: 323943