[binutils-gdb] Remove free_cached_comp_units cleanups

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*** TEST RESULTS FOR COMMIT 11ed8cada64e717900117364c2fee0132c1eb099 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 11ed8cada64e717900117364c2fee0132c1eb099

Remove free_cached_comp_units cleanups

This changes free_cached_comp_units from a cleanup function to an RAII
class.

gdb/ChangeLog
2018-03-30  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (class free_cached_comp_units): New class.
	(dw2_instantiate_symtab, dwarf2_build_psymtabs_hard): Use it.
	(free_cached_comp_units): Remove function.