GroupVarint type casting

Summary:
Pulled out one of the fixes in D63028314 into this diff. The rest of the D63028314 fixes the this-capture compiler warning-turned-error, but that's being suppressed globally (for just Mac builds) via

https://www.internalfb.com/code/fbsource/[147e83ce8584d0a790b68a5e9196e84e4834179d]/fbcode/tools/build/buck/gen_modes.py?lines=3764

Both "xbat" which is Xcode 16 clang, and "pika-16", based off a branch cut that brings pika to a clang close to Xcode 16 clang fail with this error:
```
fbcode/folly/GroupVarint.h:80:24: error: arithmetic between different enumeration types ('folly::detail::GroupVarintBase<unsigned int>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:52:3)' and 'folly::detail::GroupVarintBase<unsigned int>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:60:3)') is deprecated [-Werror,-Wdeprecated-anon-enum-enum-conversion]
   80 |     return kHeaderSize + kGroupSize + key(a) + key(b) + key(c) + key(d);
      |            ~~~~~~~~~~~ ^ ~~~~~~~~~~
fbcode/folly/GroupVarint.h:135:24: error: arithmetic between different enumeration types ('folly::detail::GroupVarintBase<unsigned int>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:52:3)' and 'folly::detail::GroupVarintBase<unsigned int>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:60:3)') is deprecated [-Werror,-Wdeprecated-anon-enum-enum-conversion]
  135 |     return kHeaderSize + kGroupSize + b0key(uint8_t(*p)) + b1key(uint8_t(*p)) +
      |            ~~~~~~~~~~~ ^ ~~~~~~~~~~
fbcode/folly/GroupVarint.h:280:24: error: arithmetic between different enumeration types ('folly::detail::GroupVarintBase<unsigned long long>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:52:3)' and 'folly::detail::GroupVarintBase<unsigned long long>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:60:3)') is deprecated [-Werror,-Wdeprecated-anon-enum-enum-conversion]
  280 |     return kHeaderSize + kGroupSize + key(a) + key(b) + key(c) + key(d) +
      |            ~~~~~~~~~~~ ^ ~~~~~~~~~~
fbcode/folly/GroupVarint.h:343:24: error: arithmetic between different enumeration types ('folly::detail::GroupVarintBase<unsigned long long>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:52:3)' and 'folly::detail::GroupVarintBase<unsigned long long>::(unnamed enum at fbcode/folly/detail/GroupVarintDetail.h:60:3)') is deprecated [-Werror,-Wdeprecated-anon-enum-enum-conversion]
  343 |     return kHeaderSize + kGroupSize + b0key(n) + b1key(n) + b2key(n) +
      |            ~~~~~~~~~~~ ^ ~~~~~~~~~~
4 errors generated.
```

This diff type-casts the offending args.

Reviewed By: Gownta

Differential Revision: D63461177

fbshipit-source-id: 89e16365baef5ba8711c0cacb9bd6e44aa25c480

folly/GroupVarint.h

@@ -77,7 +77,8 @@ class GroupVarint<uint32_t> : public detail::GroupVarintBase<uint32_t> {
    * Return the number of bytes used to encode these four values.
    */
   static size_t size(uint32_t a, uint32_t b, uint32_t c, uint32_t d) {
-    return kHeaderSize + kGroupSize + key(a) + key(b) + key(c) + key(d);
+    return (size_t)kHeaderSize + (size_t)kGroupSize + key(a) + key(b) + key(c) +
+        key(d);
   }
 
   /**
@@ -132,8 +133,8 @@ class GroupVarint<uint32_t> : public detail::GroupVarintBase<uint32_t> {
    * return the number of bytes used by the encoding.
    */
   static size_t encodedSize(const char* p) {
-    return kHeaderSize + kGroupSize + b0key(uint8_t(*p)) + b1key(uint8_t(*p)) +
-        b2key(uint8_t(*p)) + b3key(uint8_t(*p));
+    return (size_t)kHeaderSize + (size_t)kGroupSize + b0key(uint8_t(*p)) +
+        b1key(uint8_t(*p)) + b2key(uint8_t(*p)) + b3key(uint8_t(*p));
   }
 
   /**
@@ -277,8 +278,8 @@ class GroupVarint<uint64_t> : public detail::GroupVarintBase<uint64_t> {
    */
   static size_t size(
       uint64_t a, uint64_t b, uint64_t c, uint64_t d, uint64_t e) {
-    return kHeaderSize + kGroupSize + key(a) + key(b) + key(c) + key(d) +
-        key(e);
+    return (size_t)kHeaderSize + (size_t)kGroupSize + key(a) + key(b) + key(c) +
+        key(d) + key(e);
   }
 
   /**
@@ -340,8 +341,8 @@ class GroupVarint<uint64_t> : public detail::GroupVarintBase<uint64_t> {
    */
   static size_t encodedSize(const char* p) {
     uint16_t n = loadUnaligned<uint16_t>(p);
-    return kHeaderSize + kGroupSize + b0key(n) + b1key(n) + b2key(n) +
-        b3key(n) + b4key(n);
+    return (size_t)kHeaderSize + (size_t)kGroupSize + b0key(n) + b1key(n) +
+        b2key(n) + b3key(n) + b4key(n);
   }
 
   /**