Issue: 6239

This commit moves the global variables associated with engine analysis
into the detect engine context. Doing so provides encapsulation of the
analysis components as well as thread-safe operation in a multi-tenant
(context) deployment.

(cherry picked from commit c8615bc)

Where clang --version was returning:

clang version 9.0.1-15+b1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Newer version like clang-10 on Debian are returning:

Debian clang version 10.0.1-8+b1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

As a result the parsing was failing to determine which llc was
available on the system.

Ticket: OISF#6598

(cherry picked from commit 37b1595)

The commit updates obsolete installation instructions
from the userguide and reformats certain parts for
improved clarity.

Ticket: OISF#5988