Refactor DebugLocDWARFExpression so it doesn't require access to the

Refactor DebugLocDWARFExpression so it doesn't require access to the
TargetRegisterInfo. DebugLocEntry now holds a buffer with the raw bytes
of the pre-calculated DWARF expression.

Ought to be NFC, but it does slightly alter the output format of the
textual assembly.

This reapplies 230930 without the assertion in DebugLocEntry::finalize()
because not all Machine registers can be lowered into DWARF register
numbers and floating point constants cannot be expressed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231023 91177308-0d34-0410-b5e6-96231b3b80d8