Modified the ArchSpec to take an optional "Platform *" when setting the triple.

This allows you to have a platform selected, then specify a triple using
"i386" and have the remaining triple items (vendor, os, and environment) set
automatically.

Many interpreter commands take the "--arch" option to specify an architecture
triple, so now the command options needed to be able to get to the current
platform, so the Options class now take a reference to the interpreter on
construction.

Modified the build LLVM building in the Xcode project to use the new
Xcode project level user definitions:

LLVM_BUILD_DIR - a path to the llvm build directory
LLVM_SOURCE_DIR - a path to the llvm sources for the llvm that will be used to build lldb
LLVM_CONFIGURATION - the configuration that lldb is built for (Release, 
Release+Asserts, Debug, Debug+Asserts).

I also changed the LLVM build to not check if "lldb/llvm" is a symlink and
then assume it is a real llvm build directory versus the unzipped llvm.zip
package, so now you can actually have a "lldb/llvm" directory in your lldb
sources.

llvm-svn: 129112