Don't remove all indentation when in #defines.

Otherwise, this can become hard to read.

Before: #define A \
        case 1:
After:  #define A \
           case 1:

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177509 91177308-0d34-0410-b5e6-96231b3b80d8