llvm-link option and test for recent metadata mapping bug

Summary:
Add a -preserve-modules option to llvm-link that simulates LTO
clients that don't destroy modules as they are linked. This enables
reproduction of a recent bug introduced by a metadata linking change
that was only caught when the modules weren't destroyed before
writing bitcode (LTO on Windows).

See http://llvm.org/viewvc/llvm-project?view=revision&revision=253170
for more details on the original bug and the fix.

Confirmed the new test added here reproduces the failure using the new
option when I suppress the fix.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14818

llvm-svn: 253740