Revert a5c8ec4b "[CGDebugInfo] Emit subprograms for decls when AT_tail_call is understood"

This caused Chromium builds to fail with "inlinable function call in a function
with debug info must have a !dbg location" errors. See
https://bugs.chromium.org/p/chromium/issues/detail?id=1022296#c1 for a
reproducer.

> Currently, clang emits subprograms for declared functions when the
> target debugger or DWARF standard is known to support entry values
> (DW_OP_entry_value & the GNU equivalent).
>
> Treat DW_AT_tail_call the same way to allow debuggers to follow cross-TU
> tail calls.
>
> Pre-patch debug session with a cross-TU tail call:
>
> ```
>   * frame #0: 0x0000000100000fa4 main`target at b.c:4:3 [opt]
>     frame #1: 0x0000000100000f99 main`main at a.c:8:10 [opt]
> ```
>
> Post-patch (note that the tail-calling frame, "helper", is visible):
>
> ```
>   * frame #0: 0x0000000100000fa4 main`target at b.c:4:3 [opt]
>     frame #1: 0x0000000100000f80 main`helper [opt] [artificial]
>     frame #2: 0x0000000100000f99 main`main at a.c:8:10 [opt]
> ```
>
> rdar://46577651
>
> Differential Revision: https://reviews.llvm.org/D69743