Make Scalar::GetValue more consistent

Summary:
It seems the original intention of the function was printing signed values in decimal format, and
unsigned values in hex (without the leading "0x"). However, signed and unsigned long were
exchanged, which lead to amusing test failures in TestMemoryFind.py.

Instead of just switching the two, I think we should just print everything in decimal here, as
the current behaviour is very confusing (especially when one does not request printing of types).
Nothing seems to depend on this behaviour except and we already have a way for the user to
request the format he wants when printing values for most commands (which presumably does not go
through this function).

I also add a unit tests for the function in question.

Reviewers: clayborg, granata.enrico

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D24126

llvm-svn: 280476