Align a multiline string literal with the first part.

Before:
  #define A(X)          \
    "aaaaa" #X "bbbbbb" \
               "ccccc"

After:
  #define A(X)          \
    "aaaaa" #X "bbbbbb" \
    "ccccc"

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181732 91177308-0d34-0410-b5e6-96231b3b80d8