Add support for multi-file input to clang2graph

[ChrisCummins]

🚀 Feature

At the moment only a single compilation job is allowed for clang2graph. This means that generating a graph from a multifile source code requires jumping through some hoops:

# Generate .bc files
clang -c -emit-llvm *.c
# Link into a single .bc file
llvm-link *.bc -o linked.bc
# Create graph from the single .bc file
llvm2graph linked.bc

This is inconvenient and means the user loses the original module information. Instead, we should allow clang2graph to accept multiple inputs:

clang2graph *.c