[lldb/PlatformPOSIX] Change LoadImage default to RTLD_LAZY

In general, it seems like the debugger should allow programs to load & run with
libraries as far as possible, instead of defaulting to being super-picky about
unavailable symbols.

This is critical on macOS/Darwin, as libswiftCore.dylib may 1) export a version
symbol using @available markup and then 2) expect that other exported APIs are
only dynamically used once the version symbol is checked. We can't open a
version of the library built with a bleeding-edge SDK on an older OS without
RTLD_LAXY (or pervasive/expensive @available markup added to dyld APIs).

See: https://lists.llvm.org/pipermail/lldb-dev/2021-March/016796.html

Differential Revision: https://reviews.llvm.org/D98879