[DWARFExpression] Read literars as unsigned values.

[JDevlieghere]

Summary

After landing rL341457, we started seeing a failure on the swift-lldb bots. The change was correct and pretty straightforward, a DW_OP_constu was replaced with DW_OP_lit23, the value remaining identical.

0x000000f4:         DW_TAG_variable
                      DW_AT_location    (0x00000000
                         [0x0000000100000a51,  0x0000000100000d47): DW_OP_lit23, DW_OP_stack_value)
                      DW_AT_name        ("number")

However, this broke LLDB:

(Int) number = <extracting data from value failed>

The value was read correctly, but apparently the value's type was different. When reading a constu it was reading a uint64 (m_type = e_ulonglong) while for the literal, it got a signed int (m_type = e_sint). This change makes sure we read the value as an unsigned.

[clayborg]

Can we add a test for this?

[JDevlieghere]

Added a test case. I really didn't know where to put it so I decided on something generic. Hope that's fine.

[JDevlieghere]

ping

(I'd like to land this so I can un-revert the DW_OP_constu normalization for Swift)

[clayborg]

The test really should encode actual DWARF that contains a DW_OP_litXXX opcode (using obj2yaml/yaml2obj or llvm-mc) as compiling the code for the test won't always produce the needed DWARF opcode,, but the change is simple and correct.

source/Expression/DWARFExpression.cpp

     case DW_OP_lit24:
     case DW_OP_lit25:
     case DW_OP_lit26:
     case DW_OP_lit27:
     case DW_OP_lit28:
     case DW_OP_lit29:
     case DW_OP_lit30:
     case DW_OP_lit31:
-      stack.push_back(Scalar(op - DW_OP_lit0));
+      stack.push_back(Scalar((uint64_t(op - DW_OP_lit0)));
       break;
 
     //----------------------------------------------------------------------
     // OPCODE: DW_OP_regN
     // OPERANDS: none
     // DESCRIPTION: Push the value in register n on the top of the stack.
     //----------------------------------------------------------------------
     case DW_OP_reg0: