This will be used in ASTContext::getTypeInfo which needs this
information for all builtin types, not just pointers.

HLSL has a set of intangible types which are described in in the
[draft HLSL Specification
(**[Basic.types]**)](https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf):
  There are special implementation-defined types such as handle types,
  which fall into a category of standard intangible types. Intangible
  types are types that have no defined object representation or value
  representation, as such the size is unknown at compile time.
    
  A class type T is an intangible class type if it contains an base
  classes or members of intangible class type, standard intangible type,
  or arrays of such types. Standard intangible types and intangible class
  types are collectively called intangible
  types([9](https://microsoft.github.io/hlsl-specs/specs/hlsl.html#Intangible)).

This PR implements one standard intangible type `__hlsl_resource_t`
and sets up the infrastructure that will make it easier to add more
in the future, such as samplers or raytracing payload handles. The
HLSL intangible types are declared in
`clang/include/clang/Basic/HLSLIntangibleTypes.def` and this file is
included with related macro definition in most places that require edits
when a new type is added.

The new types are added as keywords and not typedefs to make sure they
cannot be redeclared, and they can only be declared in builtin implicit
headers. The `__hlsl_resource_t` type represents a handle to a memory
resource and it is going to be used in builtin HLSL buffer types like this:

        template <typename T>
        class RWBuffer {
          [[hlsl::contained_type(T)]]
          [[hlsl::is_rov(false)]]
          [[hlsl::resource_class(uav)]]  
          __hlsl_resource_t Handle;
        };

Part 1/3 of llvm/llvm-project#90631.

---------

Co-authored-by: Justin Bogner <mail@justinbogner.com>

This patch adds a new builtin type for AMDGPU's buffer rsrc data type,
which is effectively an AS 8 pointer. This is needed because we'd like
to expose certain intrinsics to users via builtins which take buffer
rsrc as argument.

This patch revolves around the misuse of UnresolvedLookupExpr in
BuildTemplateIdExpr.
    
Basically, we build up an UnresolvedLookupExpr not only for function
overloads but for "unresolved" templates wherever we need an expression
for template decls. For example, a dependent VarTemplateDecl can be
wrapped with such an expression before template instantiation. (See

https://github.com/llvm/llvm-project/commit/617007240cbfb97c8ccf6d61b0c4ca0bb62d43c9)
    
Also, one important thing is that UnresolvedLookupExpr uses a
"canonical"
QualType to describe the containing unresolved decls: a DependentTy is
for dependent expressions and an OverloadTy otherwise. Therefore, this
modeling for non-dependent templates leaves a problem in that the
expression
is marked and perceived as if describing overload functions. The
consumer then
expects functions for every such expression, although the fact is the
reverse.
Hence, we run into crashes.
    
As to the patch, I added a new canonical type "UnresolvedTemplateTy" to
model these cases. Given that we have been using this model
(intentionally or
accidentally) and it is pretty baked in throughout the code, I think
extending the role of UnresolvedLookupExpr is reasonable. Further, I
added
some diagnostics for the direct occurrence of these expressions, which
are supposed to be ill-formed.

As a bonus, this patch also fixes some typos in the diagnostics and
creates
RecoveryExprs rather than nothing in the hope of a better error-recovery
for clangd.
    
Fixes https://github.com/llvm/llvm-project/issues/88832
Fixes https://github.com/llvm/llvm-project/issues/63243
Fixes https://github.com/llvm/llvm-project/issues/48673

OpenACC is going to need an array sections implementation that is a
simpler version/more restrictive version of the OpenMP version. 

This patch moves `OMPArraySectionExpr` to `Expr.h` and renames it `ArraySectionExpr`,
 then adds an enum to choose between the two.

This also fixes a couple of 'drive-by' issues that I discovered on the way,
but leaves the OpenACC Sema parts reasonably unimplemented (no semantic
analysis implementation), as that will be a followup patch.

The IdentifierInfo isn't typically modified. Use 'const' wherever
possible.

This patch introduces a new type __externref_t that denotes a WebAssembly opaque
reference type. It also implements builtin __builtin_wasm_ref_null_extern(),
that returns a null value of __externref_t. This lays the ground work
for further builtins and reference types.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D122215

Very likely breaks stage 3 of msan build bot.
Good: 764c88a50ac76a2df2d051a0eb5badc6867aabb6 https://lab.llvm.org/buildbot/#/builders/74/builds/17058
Looks unrelated: 48b5a06dfcab12cf093a1a3df42cb5b684e2be4c
Bad: 48b5a06dfcab12cf093a1a3df42cb5b684e2be4c https://lab.llvm.org/buildbot/#/builders/74/builds/17059

This reverts commit eb66833d19573df97034a81279eda31b8d19815b.

This patch introduces a new type __externref_t that denotes a WebAssembly opaque
reference type. It also implements builtin __builtin_wasm_ref_null_extern(),
that returns a null value of __externref_t. This lays the ground work
for further builtins and reference types.

Differential Revision: https://reviews.llvm.org/D122215