[cmake] When compiling swift libraries that are not pure swift on macOS be sure to set the linker language to cxx.
[cmake] When compiling swift libraries that are not pure swift on macOS be sure to set the linker language to cxx.

I already did this for executables. The reason why we must do this is that right
now there are bugs in cmake's swift support that makes it so that one can not
use swiftc to link targets with mixed c/c++/swift content even if the swift
content is indirectly added via linking. To work around this we must also ensure
that all libraries with mixed c/c++/swift objects link using clang. As an
additional side-effect of this, we must pass to the clang linker -L flags that
swiftc would normally provide for the linker. The two cases where I needed to do
this were:

1. -L path for the compatibility libraries. This path points into the toolchain
where these live. This is important since otherwise we will fail to link given
the minimum deployment target that swift (both host/stdlib) target today (10.9).

2. -L path to the SDK. This path points to the SDKs lib/swift/${HOST_PLATFORM}
for swiftCore and friends.

That being said, we still want people to be able to link pure swift libraries
using swiftc, so we introduce an option called PURE_SWIFT to add_host_library
that preserves this and is used to also set
IGNORE_LLVM_UPDATE_COMPILE_FLAGS (which arguably should have been called
PURE_SWIFT).

JGiola/swift,rudkx/swift,JGiola/swift,xwu/swift,benlangmuir/swift,ahoppen/swift,roambotics/swift,glessard/swift,parkera/swift,ahoppen/swift,atrick/swift,glessard/swift,xwu/swift,apple/swift,gregomni/swift,tkremenek/swift,rudkx/swift,apple/swift,atrick/swift,hooman/swift,benlangmuir/swift,tkremenek/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,hooman/swift,atrick/swift,xwu/swift,hooman/swift,apple/swift,hooman/swift,ahoppen/swift,JGiola/swift,atrick/swift,gregomni/swift,tkremenek/swift,parkera/swift,roambotics/swift,glessard/swift,JGiola/swift,apple/swift,glessard/swift,benlangmuir/swift,rudkx/swift,roambotics/swift,roambotics/swift,atrick/swift,parkera/swift,parkera/swift,atrick/swift,tkremenek/swift,roambotics/swift,rudkx/swift,tkremenek/swift,rudkx/swift,gregomni/swift,hooman/swift,xwu/swift,glessard/swift,apple/swift,rudkx/swift,tkremenek/swift,glessard/swift,parkera/swift,gregomni/swift,hooman/swift,ahoppen/swift,hooman/swift,xwu/swift,gregomni/swift,parkera/swift,ahoppen/swift,tkremenek/swift,apple/swift,JGiola/swift,ahoppen/swift,parkera/swift,xwu/swift,benlangmuir/swift,xwu/swift,parkera/swift,benlangmuir/swift,gregomni/swift