Bug 1635933 - Temporarily disable new pass manager on ubsan clang-10 r=froydnj

ubsan's generated code trips an optimization corner case in clang 10. This is a short-term workaround. In the best case, llvm.org/pr45835 will land a fix soon, and we can merge that for our clang-10 update and not even land this patch. However, in case we're living in anything other than the best case world, I don't want to block on that happening.

Differential Revision: https://phabricator.services.mozilla.com/D74727

build/moz.configure/flags.configure

@@ -31,9 +31,10 @@ set_config('MOZ_RUST_DEFAULT_FLAGS', rust_flags)
 js_option('--disable-new-pass-manager',
           help='Use the legacy LLVM pass manager in clang builds')
 
-@depends('--enable-new-pass-manager', c_compiler, host, target, 'MOZ_PGO', lto.enabled,
-         enable_fuzzing)
-def new_pass_manager_flags(enabled, compiler, host, target, pgo, lto, enable_fuzzing):
+@depends('--enable-new-pass-manager', c_compiler, host, target, 'MOZ_PGO',
+         lto.enabled, enable_fuzzing, ubsan)
+def new_pass_manager_flags(enabled, compiler, host, target, pgo, lto,
+                           enable_fuzzing, ubsan):
     if host.os == 'OSX':
         # Some native Mac builds hang with the new pass manager. Given the
         # inability to test in CI, don't take the risk of further breakage.
@@ -51,6 +52,10 @@ def new_pass_manager_flags(enabled, compiler, host, target, pgo, lto, enable_fuz
     if enable_fuzzing and compiler.version < '10.0.0':
         # Clang 9 does not seem to play well with libFuzzer
         return None
+    if ubsan and compiler.version >= '10.0.0':
+        # Temporary until https://bugs.llvm.org/show_bug.cgi?id=45835 gets a
+        # real fix: clang 10 hangs with some ubsan-inserted code constructs.
+        return None
     if enabled and compiler.version >= '9.0.0':
         if compiler.type == 'clang':
             return ['-fexperimental-new-pass-manager']