Reland "[gn] port enough of 2914a4b (lldb Interfaces)"

This reverts commit 68343d7.
2914a4b relanded in bccff3b.

Author: nico

llvm/utils/gn/secondary/lldb/source/Interpreter/BUILD.gn

@@ -15,6 +15,7 @@ static_library("Interpreter") {
   deps = [
     ":InterpreterProperties",
     ":InterpreterPropertiesEnum",
+    "Interfaces",
     "//lldb/source/Commands",
     "//lldb/source/Core",
     "//lldb/source/DataFormatters",

llvm/utils/gn/secondary/lldb/source/Interpreter/Interfaces/BUILD.gn

@@ -0,0 +1,9 @@
+static_library("Interfaces") {
+  output_name = "lldbInterpreterInterfaces"
+  configs += [ "//llvm/utils/gn/build:lldb_code" ]
+  deps = [
+    "//lldb/source/Utility",
+    "//llvm/lib/Support",
+  ]
+  sources = [ "ScriptedInterfaceUsages.cpp" ]
+}