Modified lldb_private::SymboleFile to be able to override where its TypeList

comes from by using a virtual function to provide it from the Module's
SymbolVendor by default. This allows the DWARF parser, when being used to
parse DWARF in .o files with a parent DWARF + debug map parser, to get its
type list from the DWARF + debug map parser so when we go and find full 
definitions for types (that might come from other .o files), we can use the
type list from the debug map parser. Otherwise we ended up mixing clang types
from one .o file (say a const pointer to a forward declaration "class A") with
the a full type from another .o file. This causes expression parsing, when 
copying the clang types from those parsed by the DWARF parser into the 
expression AST, to fail -- for good reason. Now all types are created in the
same list.

Also added host support for crash description strings that can be set before
doing a piece of work. On MacOSX, this ties in with CrashReporter support
that allows a string to be dispalyed when the app crashes and allows 
LLDB.framework to print a description string in the crash log. Right now this
is hookup up the the CommandInterpreter::HandleCommand() where each command
notes that it is about to be executed, so if we crash while trying to do this
command, we should be able to see the command that caused LLDB to exit. For
all other platforms, this is a nop.

llvm-svn: 118672