Simplify read_unitinfo_dwarf2. Only try to read the first DIE.

Bug #305513. We should only read the first DIE of a compilation unit.
Each compilation unit header is followed by a single DW_TAG_compile_unit
(or DW_TAG_partial_unit, but those aren't important here) and its children.
There is no reason to read any of the children at this point. If the first
DIE isn't a DW_TAG_compile_unit we are done, none of the child DIEs will
provide any useful information.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13369 a5019735-40e9-0310-863c-91ae7b9d1cf9