[AST] Add UsingType: a sugar type for types found via UsingDecl

Currently there's no way to find the UsingDecl that a typeloc found its
underlying type through. Compare to DeclRefExpr::getFoundDecl().

Design decisions:
- a sugar type, as there are many contexts this type of use may appear in
- UsingType is a leaf like TypedefType, the underlying type has no TypeLoc
- not unified with UnresolvedUsingType: a single name is appealing,
  but being sometimes-sugar is often fiddly.
- not unified with TypedefType: the UsingShadowDecl is not a TypedefNameDecl or
  even a TypeDecl, and users think of these differently.
- does not cover other rarer aliases like objc @compatibility_alias,
  in order to be have a concrete API that's easy to understand.
- implicitly desugared by the hasDeclaration ASTMatcher, to avoid
  breaking existing patterns and following the precedent of ElaboratedType.

Scope:
- This does not cover types associated with template names introduced by
  using declarations. A future patch should introduce a sugar TemplateName
  variant for this. (CTAD deduced types fall under this)
- There are enough AST matchers to fix the in-tree clang-tidy tests and
  probably any other matchers, though more may be useful later.

Caveats:
- This changes a fairly common pattern in the AST people may depend on matching.
  Previously, typeLoc(loc(recordType())) matched whether a struct was
  referred to by its original scope or introduced via using-decl.
  Now, the using-decl case is not matched, and needs a separate matcher.
  This is similar to the case of typedefs but nevertheless both adds
  complexity and breaks existing code.

Differential Revision: https://reviews.llvm.org/D114251

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 4c488bc..702495df 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -2199,6 +2199,7 @@ void CodeGenFunction::EmitVariablyModifiedType(QualType type) {
     case Type::Record:
     case Type::Enum:
     case Type::Elaborated:
+    case Type::Using:
     case Type::TemplateSpecialization:
     case Type::ObjCTypeParam:
     case Type::ObjCObject: