[llvm-dwarfdump][Statistics] Ignore DW_TAG_subroutine_type in statistics

DW_TAG_subroutine_type is not really useful for statistics purposes, as it never
has location information. But it may contain DW_TAG_formal_parameter
children that generate number of parameters w/o location and decrease
'availability' metric significantly.

Reviewed by: djtodoro

Differential Revision: https://reviews.llvm.org/D72983