iwyu fixes on lldbUtility.

This patch makes adjustments to header file includes in
lldbUtility based on recommendations by the iwyu tool
(include-what-you-use).  The goal here is to make sure that
all files include the exact set of headers which are needed
for that file only, to eliminate cases of dead includes (e.g.
someone deleted some code but forgot to delete the header
includes that that code necessitated), and to eliminate the
case where header includes are picked up transitively.

llvm-svn: 299676