clang-format: Recognize single-line macro usages inside macros.

Before:
  #define LIST(L)                                                     \
    L(FirstElement) L(SecondElement) L(ThirdElement) L(FourthElement) \
        L(FifthElement)

After:
  #define LIST(L)    \
    L(FirstElement)  \
    L(SecondElement) \
    L(ThirdElement)  \
    L(FourthElement) \
    L(FifthElement)

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198407 91177308-0d34-0410-b5e6-96231b3b80d8