Support for ThinLTO function importing and symbol linking.

Summary:
Support for necessary linkage changes and symbol renaming during
ThinLTO function importing.

Also includes llvm-link support for manually importing functions
and associated llvm-link based tests.

Note that this does not include support for intelligently importing
metadata, which is currently imported duplicate times. That support will
be in the follow-on patch, and currently is ignored by the tests.

Reviewers: dexonsmith, joker.eph, davidxl

Subscribers: tobiasvk, tejohnson, llvm-commits

Differential Revision: http://reviews.llvm.org/D13515

llvm-svn: 251837