[ARM] Follow AACPS for preserving number of loads/stores of volatile bit-fields

Summary:
Following the AAPCS, every store to a volatile bit-field requires to generate one load of that field, even if all the bits are going to be replaced.
This patch allows the user to opt-in in following such rule, whenever the a.

AAPCS Release 2019Q1.1 (https://static.docs.arm.com/ihi0042/g/aapcs32.pdf)
section 8.1 Data Types, page 35, paragraph: Volatile bit-fields – preserving number and width of container accesses

```
When a volatile bit-field is written, and its container does not overlap with any non-bit-field member, its
container must be read exactly once and written exactly once using the access width appropriate to the
type of the container. The two accesses are not atomic.

```

Reviewers: lebedev.ri, ostannard, jfb, eli.friedman

Reviewed By: jfb

Subscribers: rsmith, rjmccall, dexonsmith, kristof.beyls, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67399