Install lldb's SB headers (pr36630)

Install lldb's SB headers (pr36630)

These were removed in r309021 in what looks like an accidentally
committed change. This brings them back.

I also rename the header component to lldb-headers (instead of
lldb_headers) to match the llvm style and add a special
install-lldb-headers target, which installs just the headers.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@327016 91177308-0d34-0410-b5e6-96231b3b80d8
Origin: upstream, https://github.com/llvm-mirror/lldb/commit/8f577000b2fe2f5bf5d08e352a2f15f9421f9c82.patch
Bug-Ubuntu: https://launchpad.net/bugs/1761009
Forwarded: will be done by Nishanth Aravamudan
Last-Update: 2018-04-05

Gbp-Pq: Name install-lldb-sb-headers.patch