[DwarfDebug] Fix an assertion error when emitting call site info that combines...

[DwarfDebug] Fix an assertion error when emitting call site info that combines two DW_OP_stack_values

When compiling

```
struct S {
  float w;
};
void f(long w, long b);
void g(struct S s) {
  int w = s.w;
  f(w, w*4);
}
```

I get Assertion failed: ((!CombinedExpr || CombinedExpr->isValid()) && "Combined debug expression is invalid").

That's because we combine two epxressions that both end in DW_OP_stack_value:

```
(lldb) p Expr->dump()
!DIExpression(DW_OP_LLVM_convert, 32, DW_ATE_signed, DW_OP_LLVM_convert, 64, DW_ATE_signed, DW_OP_stack_value)
(lldb) p Param.Expr->dump()
!DIExpression(DW_OP_constu, 4, DW_OP_mul, DW_OP_LLVM_convert, 32, DW_ATE_signed, DW_OP_LLVM_convert, 64, DW_ATE_signed, DW_OP_stack_value)
(lldb) p CombinedExpr->isValid()
(bool) $0 = false
(lldb) p CombinedExpr->dump()
!DIExpression(4097, 32, 5, 4097, 64, 5, 16, 4, 30, 4097, 32, 5, 4097, 64, 5, 159, 159)
```

I believe that in this particular case combining two stack values is
safe, but I didn't want to sink the special handling into
DIExpression::append() because I do want everyone to think about what
they are doing.

Patch by Adrian Prantl.

Fixes PR45181.
rdar://problem/60383095

Differential Revision: https://reviews.llvm.org/D76164