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[CGDebugInfo] Emit subprograms for decls when AT_tail_call is understood

Authored by vsk on Nov 1 2019, 5:20 PM.

Description

[CGDebugInfo] Emit subprograms for decls when AT_tail_call is understood

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