[libfuzzer] providing BUILD.gn file to simplify libfuzzer dep management.

[aizatsky]

Summary

By keeping the list of libfuzzer files in LLVM repository it is possible
to add/remove files without breaking downstream clients.

[kcc]

Have we tried all means of globing (in gn) in chromium sources?

[aizatsky]

There's only one (exec_script) and GN won't notice changes to the list of files without "ninja clean". While this might work for testdata, I'm not sure we'd like to go down this route for source files => people (and bots) would still have compilation errors.

lib/Fuzzer/BUILD.gn

+# This build file is provided for downstream gn-based clients of libfuzzer.
+# It is not a part of LLVM build sequence.
+
+source_set("libfuzzer") {
+  sources = [
+    "FuzzerCrossOver.cpp",
+    "FuzzerDriver.cpp",
+    "FuzzerFlags.def",
+    "FuzzerIO.cpp",
+    "FuzzerInterface.cpp",
+    "FuzzerLoop.cpp",
+    "FuzzerMain.cpp",
+    "FuzzerMutate.cpp",
+    "FuzzerSHA1.cpp",
+    "FuzzerSanitizerOptions.cpp",
+    "FuzzerTracePC.cpp",
+    "FuzzerTraceState.cpp",
+    "FuzzerUtil.cpp",
+  ]
+}