Fix 14934: detect out-of-bounds access to global new arrays

[JeewoongKim]

Global pointers initialized with new[] did not get a BUFFER_SIZE value because
valueFlowDynamicBufferSize() only handled allocations found inside function scopes.

This change propagates the initial buffer size into main(). The existing value-flow analysis
invalidate it after assignments or calls that may modify the global pointer.

Added regression tests for the reported out-of-bounds false negative, reassignment of the
global pointer, and an unknown function call that may modify it.

[chrchr-github]

Thanks for working on this. I'm not sure this approach is feasible though.
Global variables might be reassigned at any point, which would invalidate any previous buffer size. That's why buffer sizes are currently tracked only at the function level.
Also, this PR only handles new and not e.g. malloc().