[gn] "port" fc7857c (CIRUnitTests)

CIRUnitTests depends on mlir, so create a dummy target to make
the sync script happy. (This is behind CLANG_ENABLE_CIR in cmake.)

Author: nico

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

@@ -0,0 +1,5 @@
+# Dummy target because real CIRUnitTests depends on //mlir, which isn't
+# part of the GN build.
+group("CIRUnitTests") {
+  sources = [ "PointerLikeTest.cpp" ]
+}