[libc][__support] Fix -Wimplicit-int-conversion warning (#132839)

Newer versions of clang now warn about these, so use explicit
conversion instead.

Author: ilovepi

libc/src/__support/float_to_string.h

@@ -491,7 +491,7 @@ class FloatToString {
 
   LIBC_INLINE constexpr BlockInt get_negative_block(int block_index) {
     if (exponent < 0) {
-      const int32_t idx = -exponent / IDX_SIZE;
+      const int32_t idx = -exponent / static_cast<int32_t>(IDX_SIZE);
 
       UInt<MID_INT_SIZE> val;
 
@@ -579,7 +579,7 @@ class FloatToString {
 
     return num_requested_digits > -exponent;
 #else
-    const int32_t idx = -exponent / IDX_SIZE;
+    const int32_t idx = -exponent / static_cast<int32_t>(IDX_SIZE);
     const size_t p =
         POW10_OFFSET_2[idx] + negative_block_index - MIN_BLOCK_2[idx];
     // If the remaining digits are all 0, then this is the lowest block.
@@ -601,7 +601,7 @@ class FloatToString {
     }
     return 0;
 #else
-    return MIN_BLOCK_2[-exponent / IDX_SIZE];
+    return MIN_BLOCK_2[-exponent / static_cast<int32_t>(IDX_SIZE)];
 #endif
   }
 };