gdb/testsuite: skip gdb.cp/anon-struct.exp when using Clang

gdb/testsuite: skip gdb.cp/anon-struct.exp when using Clang

When Clang compiles anonymous structures, it does not add linkage names in
their dwarf representations. This is compounded by Clang not adding linkage
names to subprograms of those anonymous structs (for instance, the
constructor). With these 2 things together, GDB is unable to refer to
any of them, so there is no way to pass any of the tests of
gdb.cp/anon-struct.exp

Since this isn't a bug on Clang or GDB according to the DWARF
specifications as DW_AT_name is optional for all DIEs, the test was marked
as untested.

Since I was already touching the file, I also added a comment at the top
of the file explaining what it is testing for.

Approved-by: Tom Tromey <tom@tromey.com>