[libc] add exponent format to printf

Add support for the %e/E conversion in printf, as well as unit tests. It
does not yet support long doubles.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D140042

libc/src/__support/float_to_string.h

@@ -310,6 +310,15 @@ class FloatToString {
       return 0;
     }
   }
+
+  constexpr BlockInt get_block(int block_index) {
+    if (block_index >= 0) {
+      return get_positive_block(block_index);
+    } else {
+      return get_negative_block(-1 - block_index);
+    }
+  }
+
   constexpr size_t get_positive_blocks() {
     if (exponent >= -MANT_WIDTH) {
       const uint32_t idx =

libc/src/stdio/printf_core/converter.cpp

@@ -46,9 +46,9 @@ int convert(Writer *writer, const FormatSection &to_conv) {
   case 'f':
   case 'F':
     return convert_float_decimal(writer, to_conv);
-  // case 'e':
-  // case 'E':
-  // return convert_float_dec_exp(writer, to_conv);
+  case 'e':
+  case 'E':
+    return convert_float_dec_exp(writer, to_conv);
   case 'a':
   case 'A':
     return convert_float_hex_exp(writer, to_conv);