[lldb][NFCI] Remove custom dwarf LEB128 types

The LEB128 type defined by the DWARF standard is explicitly a variable-length
encoding of an integer. LLDB had defined `uleb128` and `sleb128` types
to be 32-bit  but in many places in both LLVM and LLDB we treat the maximum
width of LEB128 types to be 64, so let's remove these types and be
consistent.

Differential Revision: https://reviews.llvm.org/D150222