[vm] Fix Flutter Android builds

There seems to be some subtlety around default initialization
of std::array, which is not considered a constexpr by the combination
of libcxx and clang used in Flutter buildroot. This causes:

   error: constexpr variable 'kOffsetTo' must be initialized by a constant expression
   note: non-constexpr constructor 'array' cannot be used in a constant expression

Fix this by using explicit initialization instead.

TEST=verified manually that Flutter engine builds

Change-Id: Iaa26e846a6f19ec236529c87864d0ab4a75f6c4f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438721
Reviewed-by: Martin Kustermann <[email protected]>
Auto-Submit: Slava Egorov <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mraleph

runtime/vm/dart_entry.cc

@@ -463,8 +463,7 @@ const char* ArgumentsDescriptor::ToCString() const {
 intptr_t ArgumentsDescriptor::CacheIndexFor(intptr_t type_args_len,
                                             intptr_t num_arguments) {
   static constexpr auto kOffsetTo = []() {
-    std::array<intptr_t, kMaxTypeArgsForCachedDescriptor + 1> offset;
-    offset[0] = 0;
+    std::array<intptr_t, kMaxTypeArgsForCachedDescriptor + 1> offset = {0};
     for (intptr_t i = 1; i <= kMaxTypeArgsForCachedDescriptor; i++) {
       offset[i] =
           offset[i - 1] + (kMaxNumArgumentsForCachedDescriptor[i - 1] + 1);