re PR c/29172 (--combine can't handle #pragma once)

gcc
	PR c/29172:
	* c-opts.c (c_common_parse_file): Call cpp_clear_file_cache.
libcpp
	PR c/29172:
	* internal.h (struct cpp_reader) <file_hash_entries>: Changed
	type.
	<file_hash_entries_allocated, file_hash_entries_used>: Removed.
	* files.c (FILE_HASH_POOL_SIZE): New macro.
	(struct file_hash_entry_pool): New.
	(destroy_all_cpp_files): New function.
	(allocate_file_hash_entries): Allocate a file_hash_entry_pool.
	(new_file_hash_entry): Update.
	(free_file_hash_entries): New function.
	(_cpp_cleanup_files): Call free_file_hash_entries and
	destroy_all_cpp_files.
	(cpp_clear_file_cache): New function.
	* include/cpplib.h (cpp_clear_file_cache): Declare.

From-SVN: r130656