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

[Jeremy Morse 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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jmorse wrote:

To confirm my understanding -- what's interesting here is that the _vtable$ variable is generated in various scenarios where the vtable itself isn't generated -- thus the test is checking that _vtable$ references are generated for an external symbol?

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