[clang] Temporarily silence noisy warning for FPEvalMethod in another place

When adding an explicit default copy ctor and assignment operator,
clang emits the diag there, instead of for the class. That's narrow
enough that we can suppress the warning there too.

With this, it only files building a single file (CompilerInvocation.cpp).

See #137600 (comment)

Author: nico

clang/include/clang/Basic/LangOptions.h

@@ -76,6 +76,19 @@ class LangOptionsBase {
   using RoundingMode = llvm::RoundingMode;
   using CFBranchLabelSchemeKind = clang::CFBranchLabelSchemeKind;
 
+  LangOptionsBase() = default;
+
+#if defined(__clang__)
+// FIXME: Remove this once the warning is fixed, https://llvm.org/PR137600
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wpreferred-type-bitfield-enum-conversion"
+#endif
+  LangOptionsBase(const LangOptionsBase&) = default;
+  LangOptionsBase& operator=(const LangOptionsBase&) = default;
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
   enum GCMode { NonGC, GCOnly, HybridGC };
   enum StackProtectorMode { SSPOff, SSPOn, SSPStrong, SSPReq };