[lldb][NFC] Allow creating ClangExpressionDeclMap and ClangASTSource without a...

[lldb][NFC] Allow creating ClangExpressionDeclMap and ClangASTSource without a Target and add basic unit test

The ClangExpressionDeclMap should be testable from a unit test. This is currently
impossible as they have both dependencies on Target/ExecutionContext from their
constructor. This patch allows constructing these classes without an active Target
and adds the missing tests for running without a target that we can do at least
a basic lookup test without crashing.