[gn] Make ubsan errors fatal, as in cmake

[rnk]

Summary

Apparently ubsan errors are non-fatal by default. If you introduce UB
into LLVM and run the tests, if errors are not fatal, the test will
still produce the expected output and the tests will pass. In order to
make ubsan errors show up as test failures, they have to be made fatal.
Pass the -fno-sanitize-recover=all flag to make it so.

llvm/utils/gn/build/BUILD.gn

config("compiler_defaults") {
   }
   assert(
       !use_goma || sysroot != "",
       "goma needs a sysroot: run `llvm/utils/sysroot.py make-fake --out-dir=sysroot` and add `sysroot = \"//sysroot\"` to your args.gn")
 
   if (use_ubsan) {
     assert(is_clang && current_os == "linux",
            "ubsan only supported on Linux/Clang")
-    cflags += [ "-fsanitize=undefined" ]
+    cflags += [ "-fsanitize=undefined", "-fno-sanitize-recover=all" ]
     ldflags += [ "-fsanitize=undefined" ]
   }
 
   if (use_asan) {
     assert(is_clang && current_os == "linux",
            "asan only supported on Linux/Clang")
     cflags += [ "-fsanitize=address" ]
     ldflags += [ "-fsanitize=address" ]