[PATCH] D123345: Treat `std::move`, `forward`, and `move_if_noexcept` as builtins.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

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We still require these functions to be declared before they can be used,
but don't instantiate their definitions unless their addresses are
taken. Instead, code generation, constant evaluation, and static
analysis are given direct knowledge of their effect.

This change aims to reduce various costs associated with these functions

- per-instantiation memory costs, compile time and memory costs due to

creating out-of-line copies and inlining them, code size at -O0, and so
on -- so that they are not substantially more expensive than a cast.
Most of these improvements are very small, but I measured a 3% decrease
in -O0 object file size for a simple C++ source file using the standard
library after this change.

We now automatically infer the `const` and `nothrow` attributes on these
now-builtin functions, in particular meaning that we get a warning for
an unused call to one of these functions.

In C++20 onwards, we disallow taking the addresses of these functions,
per the C++20 "addressable function" rule. In earlier language modes, a
compatibility warning is produced but the address can still be taken.

The same infrastructure is extended to the existing MSVC builtin
`__GetExceptionInfo`, which is now only recognized in namespace `std`
like it always should have been.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123345

Files:
  clang/docs/ReleaseNotes.rst
  clang/include/clang/Basic/Builtins.def
  clang/include/clang/Basic/Builtins.h
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/AST/ExprConstant.cpp
  clang/lib/Analysis/BodyFarm.cpp
  clang/lib/Basic/Builtins.cpp
  clang/lib/CodeGen/CGBuiltin.cpp
  clang/lib/Sema/SemaChecking.cpp
  clang/lib/Sema/SemaDecl.cpp
  clang/lib/Sema/SemaExpr.cpp
  clang/lib/Sema/SemaExprCXX.cpp
  clang/lib/Sema/SemaInit.cpp
  clang/lib/Sema/SemaLookup.cpp
  clang/lib/Sema/SemaOverload.cpp
  clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
  clang/test/Analysis/use-after-move.cpp
  clang/test/CodeGenCXX/builtin-std-move.cpp
  clang/test/CodeGenCXX/microsoft-abi-throw.cpp
  clang/test/SemaCXX/builtin-std-move.cpp
  clang/test/SemaCXX/unqualified-std-call-fixits.cpp
  clang/test/SemaCXX/unqualified-std-call.cpp
  clang/test/SemaCXX/warn-consumed-analysis.cpp

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