Add -Wreturn-local-addr, GCC alias for -Wreturn-stack-address

[adriandole]

Summary

For warning compatibility with GCC.

[cjdb]

LGTM

[aaron.ballman]

LGTM as far as the functional changes go, but this should have a release note and a test.

[adriandole]

Add test and release note.

[adriandole]

Rebase

[aaron.ballman]

LGTM! Do you need someone to commit this on your behalf? If so, what name and email address would you like for patch attribution?

clang/docs/ReleaseNotes.rst

- Clang has an internal limit of 2GB of preprocessed source code per
   compilation, including source reachable through imported AST files such as
   PCH or modules. When Clang hits this limit, it now produces notes mentioning
   which header and source files are consuming large amounts of this space.
   ``#pragma clang __debug sloc_usage`` can also be used to request this report.
 - Clang no longer permits the keyword 'bool' in a concept declaration as a
   concepts-ts compatibility extension.
 - Clang now diagnoses overflow undefined behavior in a constant expression while
   evaluating a compound assignment with remainder as operand.
+- Add ``-Wreturn-local-addr``, a GCC alias for ``-Wreturn-stack-address``.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 - It's now possible to set the crash diagnostics directory through
   the environment variable ``CLANG_CRASH_DIAGNOSTICS_DIR``.
   The ``-fcrash-diagnostics-dir`` flag takes precedence.
 - When using header modules, inclusion of a private header and violations of
   the `use-declaration rules

clang/include/clang/Basic/DiagnosticGroups.td

 def LogicalNotParentheses: DiagGroup<"logical-not-parentheses">;
 def ShiftOpParentheses: DiagGroup<"shift-op-parentheses">;
 def OverloadedShiftOpParentheses: DiagGroup<"overloaded-shift-op-parentheses">;
 def DanglingElse: DiagGroup<"dangling-else">;
 def DanglingField : DiagGroup<"dangling-field">;
 def DanglingInitializerList : DiagGroup<"dangling-initializer-list">;
 def DanglingGsl : DiagGroup<"dangling-gsl">;
 def ReturnStackAddress : DiagGroup<"return-stack-address">;
+// Name of this warning in GCC
+def : DiagGroup<"return-local-addr", [ReturnStackAddress]>;
 def Dangling : DiagGroup<"dangling", [DanglingField,
                                       DanglingInitializerList,
                                       DanglingGsl,
                                       ReturnStackAddress]>;
 def DistributedObjectModifiers : DiagGroup<"distributed-object-modifiers">;
 def DllexportExplicitInstantiationDecl : DiagGroup<"dllexport-explicit-instantiation-decl">;
 def ExcessInitializers : DiagGroup<"excess-initializers">;
 def ExpansionToDefined : DiagGroup<"expansion-to-defined">;

clang/test/SemaCXX/return-stack-addr-2.cpp

 // RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -std=c++11 -Wno-return-stack-address -Wreturn-local-addr %s
 
 namespace PR26599 {
 template <typename>
 struct S;
 
 struct I {};
 
 template <typename T>