Major addition to bugpoint: ability to debug code generators (LLC and LLI).

Major addition to bugpoint: ability to debug code generators (LLC and LLI).
The C backend is assumed correct and is used to generate shared objects to be
loaded by the other two code generators.

LLC debugging should be functional now, LLI needs a few more additions to work,
the major one is renaming of external functions to call the JIT lazy function
resolver.

Bugpoint now has a command-line switch -mode with options 'compile' and
'codegen' to debug appropriate portions of tools.

ExecutionDriver.cpp: Added implementations of AbstractInterpreter for LLC and
GCC, broke out common code within other tools, and added ability to generate C
code with CBE individually, without executing the program, and the GCC tool can
generate executables shared objects or executables.

If no reference output is specified to Bugpoint, it will be generated with CBE,
because it is already assumed to be correct for the purposes of debugging using
this method. As a result, many functions now accept as an optional parameter a
shared object to be loaded in, if specified.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7293 91177308-0d34-0410-b5e6-96231b3b80d8