Fix clang narrowing error in GroupVarintTables

Summary:
The __m128i type is a pair of 64-bit signed ints and the values
generated in generate_varint_tables.py overflow this range, which
causes clang to complain.

Test Plan:
Built and ran unittests.  Also verified that the GroupVarintTable.o
data section was identical with and w/o this change.

Reviewed By: tudorb@fb.com

FB internal diff: D494645

folly/build/generate_varint_tables.py

@@ -73,7 +73,8 @@ def generate(f):
             # 0xff: set corresponding byte in result to 0
             for k in range(d, 4):
                 vals[j] |= 0xff << (8 * k)
-        f.write("  {{0x{1:08x}{0:08x}LL, 0x{3:08x}{2:08x}LL}},\n".format(*vals))
+        f.write("  {{static_cast<int64_t>(0x{1:08x}{0:08x}), "
+            "static_cast<int64_t>(0x{3:08x}{2:08x})}},\n".format(*vals))
 
     f.write("};\n"
             "\n"