clang-format: Fix child-formatting in macros.

This fixes a case where the column limit was incorrectly calculated
leading to a macro like this:

  #define A                                       \
    [] {                                          \
      xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx(        \
          xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx); \
    }

exceeding the column limit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@238182 91177308-0d34-0410-b5e6-96231b3b80d8