[clang] [Clang] Implement the core language parts of P2786 - Trivial relocation (PR #127636)

[via cfe-commits]

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@@ -1057,12 +1057,24 @@ def ext_ms_abstract_keyword : ExtWarn<
   "'abstract' keyword is a Microsoft extension">,
   InGroup<MicrosoftAbstract>;
 
+def ext_relocatable_keyword : ExtWarn<
+  "'%select{trivially_relocatable|replaceable}0_if_eligible' "
+  "keyword is a C++2c extension">,
+  InGroup<CXX26>;
+def warn_relocatable_keyword  : Warning<
+  "'%select{trivially_relocatable|replaceable}0_if_eligible' "
+  "keyword is incompatible with standards before C++2c">,
+  DefaultIgnore, InGroup<CXXPre26Compat>;
+
 def err_access_specifier_interface : Error<
   "interface types cannot specify '%select{private|protected}0' access">;
 
 def err_duplicate_class_virt_specifier : Error<
   "class already marked '%0'">;
 
+def err_duplicate_class_relocation_specifier : Error<
+  "class already marked %select{'trivially_relocatable_if_eligible'|'replaceable_if_eligible'}0">;
----------------
Sirraide wrote:

```suggestion
  "class already marked '%select{trivially_relocatable|replaceable}0_if_eligible'">;
```
nit (might as well do this the same way as above)

https://github.com/llvm/llvm-project/pull/127636