[clang] [Clang](NFC) Add coverage for VTable debug info (PR #151818)

[Tomohiro Kashiwada via cfe-commits]

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+// For CTemplate we check in case of:
+// - Implicitly instantiate whole class by up-casting:
+//   * The vtable is generated with comdat
+//   * Its '_vtable$' is generated
+// - Implicitly instantiate member function only:
+//   * The vtable is NOT generated
+//   * Its '_vtable$' is generated
+// - Define explicitly instantiation:
+//   * The vtable is generated with comdat
+//   * Its '_vtable$' is generated
+// - Declare explicitly instantiation as extern:
+//  # for COFF targets:
+//   * The vtable is declared but NOT associated with '_vtable$'
+//  # for non-COFF targets:
+//   * The vtable is declared
+//   * Its '_vtable$' is generated
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kikairoya wrote:

Yes, that's the cause of #149639, so we need to check that and to check if that doesn't happen for the COFF platform (in the next PR).

Oops, ```# for COFF targets:``` is odd here -- MinGW cases are not added yet.

https://github.com/llvm/llvm-project/pull/151818