Recommit "[lldb] Don't dissasemble large functions by default"

This recommits f665e80c which was reverted in 1cbd1b8f for breaking
TestFoundationDisassembly.py. The fix is to use --force in the test to avoid
bailing out on large functions.

I have also doubled the large function limit to 8000 bytes (~~ 2000 insns), as
the foundation library contains a lot of large-ish functions. The intent of this
feature is to prevent accidental disassembling of enormous (multi-megabyte)
"functions", not to get in people's way.

The original commit message follows:

If we have a binary without symbol information (and without
LC_FUNCTION_STARTS, if on a mac), then we have to resort to using
heuristics to determine the function boundaries. However, these don't
always work, and so we can easily end up thinking we have functions
which are several megabytes in size. Attempting to (accidentally)
disassemble these can take a very long time spam the terminal with
thousands of lines of disassembly.

This patch works around that problem by adding a sanity check to the
disassemble command. If we are about to disassemble a function which is
larger than a certain threshold, we will refuse to disassemble such a
function unless the user explicitly specifies the number of instructions
to disassemble, uses start/stop addresses for disassembly, or passes the
(new) --force argument.

The threshold is currently fairly aggressive (4000 bytes ~~ 1000
instructions). If needed, we can increase it, or even make it
configurable.

Differential Revision: https://reviews.llvm.org/D79789