Replace 'AllowExplicit' bool with an enum. No functionality change.

In passing, split it up into three values (no explicit functions /
explicit conversion functions only / any explicit functions) in
preparation for using that in a future change.
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
index 5d253cd..cb08079 100644
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -5388,7 +5388,7 @@
     ImplicitConversionSequence ICS =
         TryImplicitConversion(VariantRef, FnPtrType.getUnqualifiedType(),
                               /*SuppressUserConversions=*/false,
-                              /*AllowExplicit=*/false,
+                              AllowedExplicit::None,
                               /*InOverloadResolution=*/false,
                               /*CStyle=*/false,
                               /*AllowObjCWritebackConversion=*/false);