[gn] Make FrontendTests depend on AllTargetsDescs

The CMake build merged most clang unittests into a single binary.
The gn build doesn't (yet?) do this, but as part of that change,
FrontendTests somehow picked up a dependency on AllTargetsDescs,
see #134196 (comment)

Author: nico

llvm/utils/gn/secondary/clang/unittests/Frontend/BUILD.gn

@@ -12,6 +12,7 @@ unittest("FrontendTests") {
     "//clang/lib/Sema",
     "//clang/lib/Tooling",
     "//llvm/lib/Support",
+    "//llvm/lib/Target:AllTargetsDescs",
     "//llvm/lib/TargetParser",
   ]
   sources = [