debuginfo: Set bitwidth appropriately in enum variant tags

Previously, we unconditionally set the bitwidth to 128-bits, the largest
an enum would possibly be. Then, LLVM would cut down the constant by
chopping off leading zeroes before emitting the DWARF. LLVM only
supported 64-bit enumerators, so this would also have occasionally
resulted in truncated data.

LLVM added support for 128-bit enumerators in llvm/llvm-project#125578

That patchset also trusts the constant to describe how wide the variant tag is.
As a result, we went from emitting tags that looked like:
DW_AT_discr_value     (0xfe)

(`form1`)

to emitting tags that looked like:
DW_AT_discr_value	(<0x10> fe ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00 )

This makes the `DW_AT_discr_value` encode at the bitwidth of the tag,
which:
1. Is probably closer to our intentions in terms of describing the data.
2. Doesn't invoke the 128-bit support which may not be supported by all
   debuggers / downstream tools.
3. Will result in smaller debug information.

Author: maurer

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs

@@ -437,6 +437,11 @@ fn build_enum_variant_member_di_node<'ll, 'tcx>(
         .source_info
         .unwrap_or_else(|| (unknown_file_metadata(cx), UNKNOWN_LINE_NUMBER));
 
+    let discr = discr_value.opt_single_val().map(|value| {
+        let size = cx.size_of(tag_base_type);
+        cx.const_uint_big(cx.type_ix(size.bits()), value)
+    });
+
     unsafe {
         llvm::LLVMRustDIBuilderCreateVariantMemberType(
             DIB(cx),
@@ -448,7 +453,7 @@ fn build_enum_variant_member_di_node<'ll, 'tcx>(
             enum_type_and_layout.size.bits(),
             enum_type_and_layout.align.abi.bits() as u32,
             Size::ZERO.bits(),
-            discr_value.opt_single_val().map(|value| cx.const_u128(value)),
+            discr,
             DIFlags::FlagZero,
             variant_member_info.variant_struct_type_di_node,
         )