fix: Spurious compiler error due to deep inlining (facebookincubator#16002)

Summary:

Ubuntu release build gives the following error due to GCC's spurious deep inlining optimization when inlining creation of StringViews : 
```
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:29:33: error: ‘void* __builtin___memcpy_chk(void*, const void*, long unsigned int, long unsigned int)’ writing 13 or more bytes into a region of size 12 overflows the destination [-Werror=stringop-overflow=]
   29 |   return __builtin___memcpy_chk (__dest, __src, __len,
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
   30 |                                  __glibc_objsize0 (__dest));
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~
```

This fixes it by letting the compiler know that its not possible to have length > 12 in the inlining case. 
Fixes facebookincubator#15245

Differential Revision: D90620860

Author: Krishna Pai

velox/type/StringView.h

@@ -92,7 +92,17 @@ struct StringView {
       // small string: inlined. Zero the last 8 bytes first to allow for whole
       // word comparison.
       value_.data = nullptr;
+#ifdef __GNUC__
+      // Use asm volatile to prevent GCC's interprocedural analysis from
+      // incorrectly concluding that size could exceed kInlineSize. This is a
+      // workaround for a false positive -Wstringop-overflow warning when GCC
+      // cannot prove the size constraint through deep template inlining.
+      auto sz = static_cast<size_t>(size_);
+      asm volatile("" : "+r"(sz));
+      memcpy(prefix_, data, sz);
+#else
       memcpy(prefix_, data, size_);
+#endif
     } else {
       // large string: store pointer
       memcpy(prefix_, data, kPrefixSize);