IR: Enable debug info type ODR uniquing for forward decls

Add a new method, DICompositeType::buildODRType, that will create or
mutate the DICompositeType for a given ODR identifier, and use it in
LLParser and BitcodeReader instead of DICompositeType::getODRType.

The logic is as follows:

  - If there's no node, create one with the given arguments.
  - Else, if the current node is a forward declaration and the new
    arguments would create a definition, mutate the node to match the
    new arguments.
  - Else, return the old node.

This adds a missing feature supported by the current DITypeIdentifierMap
(which I'm slowly making redudant).  The only remaining difference is
that the DITypeIdentifierMap has a "the-last-one-wins" rule, whereas
DICompositeType::buildODRType has a "the-first-one-wins" rule.

For now I'm leaving behind DICompositeType::getODRType since it has
obvious, low-level semantics that are convenient for unit testing.

llvm-svn: 266786

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2207,8 +2207,8 @@ std::error_code BitcodeReader::parseMetadata(bool ModuleLevel) {
       Metadata *TemplateParams = getMDOrNull(Record[14]);
       auto *Identifier = getMDString(Record[15]);
       DICompositeType *CT = nullptr;
-      if (!(Flags & DINode::FlagFwdDecl) && Identifier)
-        CT = DICompositeType::getODRType(
+      if (Identifier)
+        CT = DICompositeType::buildODRType(
             Context, *Identifier, Tag, Name, File, Line, Scope, BaseType,
             SizeInBits, AlignInBits, OffsetInBits, Flags, Elements, RuntimeLang,
             VTableHolder, TemplateParams);