Bug 1879225: Disable narrowing warning for irregexp r=dminor

This is necessary because of an awkward bit of code [here in regexp-nodes.h](https://github.com/v8/v8/blob/ed3c1170334fc215917de4409750d22f484475f1/src/regexp/regexp-nodes.h#L360-L363) that uses a struct initializer to create a RegExpFlags. C++ initializers do not do type conversions, so if we initialize RegExpFlags with an int, then the member field of RegExpFlags must also be an int. V8's RegExpFlags is backed by an int, but we use a uint8_t.

I based the clang/gcc flags on the #pragmas [here](https://searchfox.org/mozilla-central/source/mozglue/tests/TestPrintf.cpp#14-20).

Differential Revision: https://phabricator.services.mozilla.com/D201920

js/src/irregexp/moz.build

@@ -14,9 +14,13 @@ include("../js-cxxflags.mozbuild")
 
 CXXFLAGS += ["-Wno-error=type-limits", "-Wno-error=return-type"]
 
-# Suppress spurious warnings in third-party code. See bug 1810584.
+# Suppress spurious warnings in third-party code.
+# See bug 1810584 and bug 1879225.
 if CONFIG["CC_TYPE"] == "gcc":
-    CXXFLAGS += ["-Wno-error=nonnull"]
+    CXXFLAGS += ["-Wno-error=nonnull", "-Wno-narrowing"]
+if CONFIG["CC_TYPE"] == "clang":
+    CXXFLAGS += ["-Wno-c++11-narrowing"]
+
 
 UNIFIED_SOURCES += [
     "imported/regexp-bytecode-generator.cc",