Implement trampoline step-through for Windows-x86.

Summary:
This is required to be able to step through calls to external functions
that are not properly marked with __declspec(dllimport). When a call
like this is emitted, the linker will inject a trampoline to produce an
indirect call through the IAT.

Reviewers: zturner, jingham

Reviewed By: jingham

Subscribers: sas, jingham, lldb-commits

Differential Revision: https://reviews.llvm.org/D22231

llvm-svn: 314045