clang-format: Revamp nested block formatting.

This fixed llvm.org/PR21804 and hopefully a few other strange cases.

Before:
    if (blah_blah(whatever, whatever, [] {
      doo_dah();
      doo_dah();
    })) {
    }
    }

After:
    if (blah_blah(whatever, whatever, [] {
          doo_dah();
          doo_dah();
        })) {
    }
    }
Make File always take the ownership of a MemoryBuffer.

The documentation of parseFile() said that "the resulting File
object may take ownership of the MemoryBuffer." So, whether or not
the ownership of a MemoryBuffer would be taken was not clear.
A FileNode (a subclass of InputElement, which is being deprecated)
keeps the ownership if a File doesn't take it.

This patch makes File always take the ownership of a buffer.
Buffers lifespan is not always the same as File instances.
Files are able to deallocate buffers after parsing the contents.
Silence gcc 4.9.1 warning 'xyz' is used uninitialized in this function.

In release builds this is actually possible as without asserts there is
no testing of the actual read bytes and the variables could be partially
uninitialized.