Mostly NFC. These changes are in preparation for the work done in D74053.
Commit ed29dbaafa49 removed the WasIndirect field from DbgValueLocation so
I've removed some checks that were hanging around and tidied up the class:
- Fix locNo() method: it no longer needs to work around the WasIndirect bitfield.
- Remove assertions from constructor: it no longer needs to check the bitfield packing.
- Use defaulted default constructor and in-class member initializer values.