[lldb] Add MockMemory class for dwarf expression testing (#168467)

This change unifies the way that we specify mocked memory to make it
easy to control the process and target memory contents for unit tests.
We add a MockMemory class that can be used in dwarf expression testing
to specify the output of the `ReadMemory` function.

The MockMemory class is built on a map that maps a `(address, size)`
pair to a vector of bytes that is `size` bytes long and contains the
memory contents for that `address`.

The MockProcessWithMemRead and MockTarget classes are updated to use the
new MockMemory interface. The MockProcessWithMemRead class was renamed
to MockProcess and the old MockProcess was deleted. The old MockProcess had
and ReadMemory implementation that returned the value `i & 0xff` for reading the
address `i` and was easily be replaced with the MockMemory object.

The CreateTestContext function now takes optional values for process memory and 
target memory and uses those to create the mock objects.

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