Sample profiles - Re-arrange binary format to emit head samples only on top functions.

The number of samples collected at the head of a function only make
sense for top-level functions (i.e., those actually called as opposed to
being inlined inside another).

Head samples essentially count the time spent inside the function's
prologue.  This clearly doesn't make sense for inlined functions, so we
were always emitting 0 in those.

llvm-svn: 250539