[clang-format] Support breaking consecutive string literals for TableGen

Summary:
clang-format can get confused by string literals in TableGen: it knows that strings can be broken up, but doesn't seem to understand how that can be indented across line breaks, and arranges them in a weird triangular pattern. Take this output example from `clang-format tools/llvm-objcopy/ObjcopyOpts.td` (which has now been formatted in rL345896 with this patch applied):

```
defm keep_global_symbols
: Eq<
      "keep-global-symbols", "Reads a list of symbols from <filename> and "
                             "runs as if " "--keep-global-symbol=<symbol> "
                                           "is set for each one. "
                                           "<filename> " "contains one "
                                                         "symbol per line "
                                                         "and may contain "
                                                         "comments "
                                                         "beginning " "with"
                                                                      " '#'"
                                                                      ". "
                                                                      "Lead"
                                                                      "ing "
```

Reviewers: alexshap, MaskRay, djasper

Reviewed By: MaskRay

Subscribers: krasimir, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D53952

llvm-svn: 346687