Revert "Allow signposts to take advantage of deferred string substitution"

This reverts commits f9aba9a5 and
035217ff.

As explained in the original commit message, this didn't have the
intended effect of improving the common LLDB use case, but still
provided a marginal improvement for the places where LLDB creates a
scoped time with a string literal.

The reason for the revert is that this change pulls in the os/signpost.h
header in Signposts.h. The former transitively includes loader.h, which
contains a series of macro defines that conflict with MachO.h. There are
ways to work around that, but Adrian and I concluded that  none of them
are worth the trade-off in complicating Signposts.h even further.