Fix formatter dropping function ports when packed dims contain $clog2()

[kbrunham-intel]

Summary

• Function headers whose return packed dimensions contain a system call such as $clog2(...) used to lose their port list: ReshapeFittingSubpartitions expected [header, args, trailer] and treated extra $clog2 fragments as the args, discarding the real ports.
• Collapse those extra header fragments before reshaping so the ports are kept and wrapped normally.
• Adds formatter regression tests for the original Formatting long function header removes part of code #886 sample, $bits(), multi-arg $clog2(), and extern prototypes.

Fixes #886

Test plan

• bazel test -c opt //verible/common/formatting:token-partition-tree_test //verible/verilog/formatting:formatter_test
• Original Formatting long function header removes part of code #886 sample keeps both function arguments
• CI on this PR is green

[kbrunham-intel]

Hi @hzeller. Please consider this change which fixes issue 886.