[llvm] [ArgumentPromotion] Emit DW_OP_LLVM_implicit_pointer for promoted pointer args (PR #187642)

[via llvm-commits]

yonghong-song wrote:

@phyBrackets Thanks for working on this!
I tried the following example,

```
$ cat foo.c
struct t {int a; int b;};
__attribute__((noinline)) static int foo(struct t *p) { return p->b + 5; }
int bar(int a, int b) { struct t x = {a, b}; return foo(&x); }
```
and compile with
```
clang -O3 -g -c foo.c -mllvm -print-after-all
```

Now we don't have the following
```
; *** IR Dump After ArgumentPromotionPass on (foo) ***
; Function Attrs: noinline nounwind uwtable
define internal fastcc range(i32 -2147483643, -2147483648) i32 @foo(i32 %0) unnamed_addr #2 !dbg !13 {
    #dbg_value(i32 %0, !23, !DIExpression(DW_OP_LLVM_implicit_pointer), !24)
  %2 = add nsw i32 %0, 5, !dbg !25
  ret i32 %2, !dbg !26
}

!23 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !21, file: !1, line: 1, baseType: !16, size: 32)
!24 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !21, file: !1, line: 1, baseType: !16, size: 32, offset: 32)
```

Could you explain what does the following mean?
```
#dbg_value(i32 %0, !23, !DIExpression(DW_OP_LLVM_implicit_pointer), !24)
```
and how it relates to 'p->b'?

And `llvm-dwarfdump foo.o`
```
...
0x00000061:   DW_TAG_subprogram                                                                                                                 
                DW_AT_low_pc    (0x0000000000000010)                                                                                            
                DW_AT_high_pc   (0x0000000000000014)                                                                                            
                DW_AT_frame_base        (DW_OP_reg7 RSP)                                                                                        
                DW_AT_call_all_calls    (true)      
                DW_AT_name      ("foo")                 
                DW_AT_decl_file ("/home/yhs/tests/sig-change-pahole/prom/foo.c")                                                                
                DW_AT_decl_line (2)                                     
                DW_AT_prototyped        (true)                                                                                                  
                DW_AT_calling_convention        (DW_CC_nocall)          
                DW_AT_type      (0x00000084 "int")
                                                                        
0x00000071:     DW_TAG_formal_parameter                                 
                  DW_AT_location        (DW_OP_implicit_pointer 0x0 +0) 
                  DW_AT_name    ("p")                                                                                                           
                  DW_AT_decl_file       ("/home/yhs/tests/sig-change-pahole/prom/foo.c")                                                        
                  DW_AT_decl_line       (2)                                                                                                     
                  DW_AT_type    (0x000000a0 "t *")                                                                                              
                                                                        
0x00000080:     NULL 
...
```

I am not sure how the above dwarf can relate to `p->b`.
Did I miss anything?

https://github.com/llvm/llvm-project/pull/187642