[lldb] Define lldbassert based on NDEBUG instead of LLDB_CONFIGURATION_DEBUG

Whether assertions are enabled or not is orthogonal to the build type
which could lead to surprising behavior for lldbassert. Previously, when
doing a debug build with assertions disabled, lldbassert would become a
NOOP, rather than printing an error like it does in a release build. By
definining lldbassert in terms of NDEBUG, it behaves like a regular
assert when assertions are enabled, and like a soft assert.

Differential revision: https://reviews.llvm.org/D150639