Only break string literals as a last resort.

We might want to move towards doing this if the formatting can be
significantly improved, but we need to carefully evaluate the different
situations first.

Before (the string literal was split by clang-format here):
aaaaaaaaaaaaaaaaaaaa(aaaaaaaaaaaaaaaaaaaa, aaaaaa("aaa aaaaa aaa aaa aaaaa aaa "
                                                  "aaaaa aaa aaa aaaaaa"));

After:
aaaaaaaaaaaaaaaaaaaa(
    aaaaaaaaaaaaaaaaaaaa,
    aaaaaa("aaa aaaaa aaa aaa aaaaa aaa aaaaa aaa aaa aaaaaa"));

llvm-svn: 176084