[lldb-dap] Don't call GetNumChildren on non-indexed synthetic variables (#93534)

A synthetic child provider might need to do considerable amount of work
to compute the number of children. lldb-dap is currently calling that
for all synthethic variables, but it's only actually using the value for
values which it deems to be "indexed" (which is determined by looking at
the name of the first child). This patch reverses the logic so that
GetNumChildren is only called for variables with a suitable first child.