[lldb/DataExtractor] Fix UB shift in GetMaxS64Bitfield

DataExtractor::GetMaxS64Bitfield performs a shift with UB in order to
construct a bitmask when bitfield_bit_size is 64. The current
implementation actually does “work” in this case, because the assumption
that the shift result is 0 holds, and 0 minus 1 gives the all-ones value
(the correct mask). However, the more readable/maintainable approach
might be to use an off-the-shelf UB-free helper.

Fixes a UBSan issue:

  "col" : 37,
  "description" : "invalid-shift-exponent",
  "filename" : "/Users/vsk/src/llvm-project-master/lldb/source/Utility/DataExtractor.cpp",
  "instrumentation_class" : "UndefinedBehaviorSanitizer",
  "line" : 615,
  "memory_address" : 0,
  "summary" : "Shift exponent 64 is too large for 64-bit type 'uint64_t' (aka 'unsigned long long')",

rdar://59117758

Differential Revision: https://reviews.llvm.org/D73913