This patch stops lldb from loading a .lldbinit file from the current

working directory by default -- a typical security problem that we
need to be more conservative about.

It adds a new target setting, target.load-cwd-lldbinit which may
be true (always read $cwd/.lldbinit), false (never read $cwd/.lldbinit)
or warn (warn if there is a $cwd/.lldbinit and don't read it).  The
default is set to warn.  If this is met with unhappiness, we can look
at changing the default to true (to match current behavior) on a 
different platform.

This does not affect reading of ~/.lldbinit - that will still be read,
as before.  If you run lldb in your home directory, it will not warn
about the presence of a .lldbinit file there.

I had to add two SB API - SBHostOS::GetUserHomeDirectory and 
SBFileSpec::AppendPathComponent - for the lldb driver code to be
able to get the home directory path in an OS neutral manner.

The warning text is

There is a .lldbinit file in the current directory which is not being read.
To silence this warning without sourcing in the local .lldbinit,
add the following to the lldbinit file in your home directory:
    settings set target.load-cwd-lldbinit false
To allow lldb to source .lldbinit files in the current working directory,
set the value of this variable to true.  Only do so if you understand and
accept the security risk.

<rdar://problem/24199163> 

llvm-svn: 261280