PR10461: Probe identical functions only once

PR10461: Probe identical functions only once

In C++ especially, a function definition in a header may be compiled
into multiple CUs, but the linker will merge those into a single output
function.  We don't want to place multiple probes on the same function.

The dupe-detection from the alias code (commit 1c6b77e5) already tracks
identical functions within a CU, so I've just lifted this to instead
track function entrypcs at the module level.

* dwflpp.cxx (dwflpp::iterate_over_functions): Remove dupe checks.
* tapsets.cxx (dwarf_query): Add alias_dupes set to the query.
  (dwarf_query::handle_query_module): Reset the dupes for each module.
  (query_dwarf_func): Check that we only probe each entrypc once.