gn build: Add build file for dexp

[thakis]

Summary

None of check-clang-tools's tests run this, but the CMake check-clang-tools depends on the binary, so add it for consistency.

[mbonadei]

LGTM.

llvm/utils/gn/secondary/clang-tools-extra/clangd/index/dex/dexp/BUILD.gn

+executable("dexp") {
+  configs += [ "//llvm/utils/gn/build:clang_code" ]
+  deps = [
+    "//clang-tools-extra/clangd",
+    "//clang/lib/Basic",
+    "//llvm/lib/LineEditor",
+    "//llvm/lib/Support",
+  ]
+  include_dirs = [ "../../.." ]
+  sources = [
+    "Dexp.cpp",
+  ]
+}

llvm/utils/gn/secondary/clang-tools-extra/test/BUILD.gn

deps = [
     "//clang-tools-extra/clang-change-namespace/tool:clang-change-namespace",
     "//clang-tools-extra/clang-doc/tool:clang-doc",
     "//clang-tools-extra/clang-include-fixer/find-all-symbols/tool:find-all-symbols",
     "//clang-tools-extra/clang-include-fixer/tool:clang-include-fixer",
     "//clang-tools-extra/clang-move/tool:clang-move",
     "//clang-tools-extra/clang-query/tool:clang-query",
     "//clang-tools-extra/clang-reorder-fields/tool:clang-reorder-fields",
     "//clang-tools-extra/clang-tidy/tool:clang-tidy",
+    "//clang-tools-extra/clangd/index/dex/dexp",
     "//clang-tools-extra/clangd/indexer:clangd-indexer",
     "//clang-tools-extra/clangd/tool:clangd",
     "//clang-tools-extra/modularize",
     "//clang-tools-extra/pp-trace",
     "//clang-tools-extra/unittests",
     "//clang/lib/Headers",
     "//clang/tools/c-index-test",
     "//clang/tools/clang-rename",
     "//clang/tools/driver:symlinks",
     "//llvm/tools/llvm-bcanalyzer",
     "//llvm/utils/FileCheck",
     "//llvm/utils/count",
     "//llvm/utils/llvm-lit",
     "//llvm/utils/not",
   ]
   if (clangd_build_xpc) {
     deps +=
         [ "//clang-tools-extra/clangd/xpc/test-client:clangd-xpc-test-client" ]
   }
-
-  # FIXME: dep on dexp once it exist
   testonly = true
 }
 
 action("check-clang-tools") {
   script = "$root_out_dir/bin/llvm-lit"
   if (host_os == "win") {
     script += ".py"
   }