Reduce memory usage for storing LTO decl resolutions

Reduce memory usage for storing LTO decl resolutions

With a LTO build of a large project (>11k subfiles incrementially linked)
storing the LTO resolutions took over 0.5GB memory:

lto/lto.c:1087 (lto_resolution_read)                      0: 0.0%  540398500           15903: 0.0%

The reason is that the declaration indexes are quite sparse, but every subfile
got a full continuous vector for them. Since there are so many of them the
many vectors add up.

This patch instead stores the resolutions initially in a compact (index, resolution)
format. This is only expanded into a sparse vector for fast lookup when
the subfile is actually read, but then immediately freed. This means only one
vector is allocated at a time.

This brings the overhead for this down to less than 3MB for the test case:

lto/lto.c:1087 (lto_resolution_read)                      0: 0.0%    2821456           42186: 0.0%

gcc/:

2012-09-06  Andi Kleen  <ak@linux.intel.com>

	* gcc/lto-streamer.h (res_pair): Add.
	(lto_file_decl_data): Replace resolutions with respairs.
	Add max_index.
	* gcc/lto/lto.c (lto_resolution_read): Remove max_index.  Add rp.
	Initialize respairs.
	(lto_file_finalize): Set up resolutions vector lazily from respairs.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@191051 138bc75d-0d04-0410-961f-82ee72b054a4