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

This patch:

* fixes an incorrect sign-extension of unsigned values, when emitting
  debug info metadata for enumerators
* the enumerators metadata is created with a flag, which determines
  interpretation of the value bits (signed or unsigned)
* the enumerations metadata contains the underlying integer type and a
  flag, indicating whether this is a C++ "fixed enum"

Differential Revision: https://reviews.llvm.org/D42736

llvm-svn: 324490

clang/test/Modules/ModuleDebugInfo.cpp

@@ -48,7 +48,7 @@
 // CHECK: !DICompositeType(tag: DW_TAG_enumeration_type,
 // CHECK-NOT:              name:
 // CHECK-SAME:             )
-// CHECK: !DIEnumerator(name: "e5", value: 5)
+// CHECK: !DIEnumerator(name: "e5", value: 5, isUnsigned: true)
 
 // CHECK: !DIDerivedType(tag: DW_TAG_typedef, name: "B",
 // no mangled name here yet.