[PATCH] Add Position-independent Code model Module API.

[Rafael Ávila de Espíndola]

This should probably include a test showing what llvm-link does when merging these.

================
Comment at: tools/llc/llc.cpp:290
@@ -289,1 +289,3 @@
 
+  mod->setPICLevel(PLLevel);
+
----------------
Is it intentional that any pre-existing metadata is always discarded?

http://reviews.llvm.org/D5882