[clang] [clang-tools-extra] [Clang][AST] Introduce `ExplicitInstantiationDecl` to preserve source info and fix diagnostic locations (PR #191658)

[Younan Zhang via cfe-commits]

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@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// Verify that the "previous explicit instantiation" note points to the first
+// explicit instantiation statement, not to the implicit instantiation site.
+// This is a regression test for https://github.com/llvm/llvm-project/issues/21133
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zyn0217 wrote:

We usually wrap a regression within a namespace, with the name pointing to the issue number. i.e. it should be namespace GH21133 here.

https://github.com/llvm/llvm-project/pull/191658