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

[Younan Zhang via cfe-commits]

================
@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -ast-print %s -o - -std=c++17 | FileCheck %s
+
+namespace ns {
+template <typename T> struct S { T x; };
+template <typename T> void foo(T) {}
+template <typename T> T var = T();
+}
+
+// Class template explicit instantiation.
----------------
zyn0217 wrote:

Can we get rid of these comments? They look useless

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