[clang-format] Do not break before long string literals in protos

[clang-format] Do not break before long string literals in protos

Summary:
This patch is a follow-up to r323319 (which disables string literal breaking for
text protos) and it disables breaking before long string literals.

For example this:
```
keyyyyy: "long string literal"
```
used to get broken into:
```
keyyyyy:
    "long string literal"
```

While at it, I also enabled it for LK_Proto and fixed a bug in the mustBreak code.

Reviewers: djasper, sammccall

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42957

llvm-svn: 324591