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

[Erich Keane 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' "
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erichkeane wrote:

Please put the `_if_eligible` inside the select.  I get that it results in duplication, but having full-keywords in diagnostics makes them easier to search and makes this less awkward.  

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