[DebugInfo] Improvements to representation of enumeration types (PR36168)

This patch is the LLVM part of fixing the issues, described in
https://bugs.llvm.org/show_bug.cgi?id=36168

* The representation of enumerator values in the debug info metadata now
  contains a boolean flag isUnsigned, which determines how the bits of
  the value are interpreted.
* The DW_TAG_enumeration type DIE now always (for DWARF version >= 3)
  includes a DW_AT_type attribute, which refers to the underlying
  integer type, as suggested in DWARFv4 (5.7 Enumeration Type Entries).
* The debug info metadata for enumeration type contains (in flags)
  indication whether this is a C++11 "fixed enum".
* For C++11 enumeration with a fixed underlying type, the DIE also
  includes the DW_AT_enum_class attribute (for DWARF version >= 4).
* Encoding of enumerator constants uses DW_FORM_sdata for signed values
  and DW_FORM_udata for unsigned values, as suggested by DWARFv4 (7.5.4
  Attribute Encodings).

The changes should be backwards compatible:

* the isUnsigned attribute is optional and defaults to false.
* if the underlying type for the enumeration is not available, the
  enumerator values are considered signed.
* the FixedEnum flag defaults to clear.
* the bitcode format for DIEnumerator stores the unsigned flag bit #1 of
  the first record element, so the format does not change and the zero
  previously stored there is consistent with the false default for
  IsUnsigned.

Differential Revision: https://reviews.llvm.org/D42734

llvm-svn: 324489

lib/Bitcode/Reader/MetadataLoader.cpp

@@ -1200,10 +1200,11 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     if (Record.size() != 3)
       return error("Invalid record");
 
-    IsDistinct = Record[0];
+    IsDistinct = Record[0] & 1;
+    bool IsUnsigned = Record[0] & 2;
     MetadataList.assignValue(
         GET_OR_DISTINCT(DIEnumerator, (Context, unrotateSign(Record[1]),
-                                       getMDString(Record[2]))),
+                                       IsUnsigned, getMDString(Record[2]))),
         NextMetadataNo);
     NextMetadataNo++;
     break;