[Clang][DebugInfo][AMDGPU] Emit zero size bitfields in the debug info to...

[Clang][DebugInfo][AMDGPU] Emit zero size bitfields in the debug info to delimit bitfields in different allocation units.

Consider the following sturctures when targetting:

  struct foo {
    int space[4];
    char a : 8;
    char b : 8;
    char x : 8;
    char y : 8;
  };

  struct bar {
    int space[4];
    char a : 8;
    char b : 8;
    char : 0;
    char x : 8;
    char y : 8;
  };

Even if both structs have the same layout in memory, they are handled
differenlty by the AMDGPU ABI.

With the following code:

// clang --target=amdgcn-amd-amdhsa -g -O1 example.c -S
char use_foo(struct foo f) { return f.y; }
char use_bar(struct bar b) { return b.y; }

For use_foo, the 'y' field is passed in v4
; v_ashrrev_i32_e32 v0, 24, v4
; s_setpc_b64 s[30:31]

For use_bar, the 'y' field is passed in v5
; v_bfe_i32 v0, v5, 8, 8
; s_setpc_b64 s[30:31]

To make this distinction, we record a single 0-size bitfield for every member that is preceded
by it.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D144870