switch to clang-format-3.8 (#2547)

https://llvm.org/bugs/show_bug.cgi?id=30353 was filed for the unintended
line break between in e.g. “TAILQ_ENTRY(foo)\nbar;”.

Until that’s fixed or a workaround is known, we’ll live with line
breaks. To make it a bit easier for readers to see what’s going on, I
added extra line breaks around each such struct member/variable
definition, so that they at least visually are a single unit.

fixes #2174

src/restore_layout.c

@@ -29,7 +29,8 @@ typedef struct placeholder_state {
     /** The graphics context for “pixmap”. */
     xcb_gcontext_t gc;
 
-    TAILQ_ENTRY(placeholder_state) state;
+    TAILQ_ENTRY(placeholder_state)
+    state;
 } placeholder_state;
 
 static TAILQ_HEAD(state_head, placeholder_state) state_head =