clang-format: Fix corner case with comment in ctor initializer.

Formatting:
  Constructor() :
      // Comment forcing unwanted break.
      aaaa(aaaa) {}

Before:
  Constructor()
      :
        // Comment forcing unwanted break.
        aaaa(aaaa) {}

After:
  Constructor()
      : // Comment forcing unwanted break.
        aaaa(aaaa) {}

llvm-svn: 199107