Remove some explicit (Mutable)ArrayRef constructions (#4238)

Rely on implicit conversion in call sites and initialization.

Removing the explicit conversions is only code simplication.
Moving from `auto x = Y(z)` to `Y x = z;` helps ensure that only
implicit constructors/conversions are happening (whereas the prior
syntax allows explicit conversions) which can help with readability
since implicit conversions are generally "less
complex"/risky/attention-requiring.

common/raw_hashtable_metadata_group_benchmark.cpp

@@ -122,7 +122,7 @@ static auto BuildBenchMetadata() -> llvm::ArrayRef<BenchMetadata> {
 
     for (ssize_t g_index : llvm::seq<ssize_t>(0, BenchSize)) {
       // Start by filling the group with random bytes.
-      auto group_bytes = llvm::MutableArrayRef(
+      llvm::MutableArrayRef group_bytes(
           &metadata_storage[bm_index][g_index * GroupSize], GroupSize);
       for (uint8_t& b : group_bytes) {
         b = absl::Uniform<uint8_t>(gen) | MetadataGroup::PresentMask;