[CMake] Make liblldb depend on clang-headers
[CMake] Make liblldb depend on clang-headers

Summary:
The clang headers are useful when dealing with clang modules. There is also a
way to get to the clang headers from the SB API so it would be nice if they were
also available when we just build lldb.

Differential Revision: https://reviews.llvm.org/D58793

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@355149 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb