[PATCH] Speculatively instantiate archive members

LLD parses archive file index table only at first. When it finds a symbol
it is looking for is defined in a member file in an archive file, it actually
reads the member from the archive file. That's done in the core linker.

That's a single-thread process since the core linker is single threaded.
If your command line contains a few object files and a lot of archive files
(which is quite often the case), LLD hardly utilizes hardware parallelism.

This patch improves parallelism by speculatively instantiating archive
file members. At the beginning of the core linking, we first create a map
containing all symbols defined in all members, and each time we find a
new undefined symbol, we instantiate a member file containing the
symbol (if such file exists). File instantiation is side effect free, so this
should not affect correctness.

This is a quick benchmark result. Time to link self-link LLD executable:

Linux   9.78s -> 8.50s (0.86x)
Windows 6.18s -> 4.51s (0.73x)

http://reviews.llvm.org/D7015

llvm-svn: 226336