[DWARFFormValue] Cleanup DWARFFormValue interface. (NFC)

[DWARFFormValue] Cleanup DWARFFormValue interface. (NFC)

DWARFFormValues can be created from a data extractor or by passing its
value directly. Until now this was done by member functions that
modified an existing object's internal state. This patch replaces a
subset of these methods with static method that return a new
DWARFFormValue.

llvm-svn: 354941