Bug 1428541 - Make char16ptr_t::operator bool() explicit. r=froydnj

clang-cl fails to build without this change due to ambiguous call.

MozReview-Commit-ID: 22x5PCsG221

--HG--
extra : rebase_source : 93d2d50abc126286ad78c1f3700fe358c507a826
extra : intermediate-source : a9c64eaa326a2c694456d2e8907074f75b66c645
extra : source : c0b55d2093f59a8dabf2640106befa0dae516906

mfbt/Char16.h

@@ -66,7 +66,7 @@ public:
   {
     return mPtr;
   }
-  MOZ_IMPLICIT operator bool() const
+  explicit operator bool() const
   {
     return mPtr != nullptr;
   }