[lto] added getCOFFWeakExternalFallback
[lto] added getCOFFWeakExternalFallback

Summary: This allows clients of the LTO API to determine the name of the fallback symbol for COFF weak externals.

Reviewers: pcc

Reviewed By: pcc

Subscribers: mehdi_amini

Differential Revision: https://reviews.llvm.org/D29365



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@293960 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm