Garbage collect dllimported symbols.

Summary:
Previously, the garbage collector (enabled by default or by explicitly
passing /opt:ref) did not kill dllimported symbols. As a result,
dllimported symbols could be added to resulting executables' dllimport
list even if no one was actually using them.

This patch implements dllexported symbol garbage collection. Just like
COMDAT sections, dllimported symbols now have Live bits to manage their
liveness, and MarkLive marks reachable dllimported symbols.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32950

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33264

llvm-svn: 303225