[Support] Add -polly-dump-module pass.

This pass allows writing the LLVM-IR just before and after the Polly
passes to a file.

Dumping the IR before Polly helps reproducing bugs that occur in code
generated by clang. It is the only reliable way to get the IR that
triggers a bug. The alternative is to emit the IR with

    clang -c -emit-llvm -S -o dump.ll

then pass it through all optimization passes

    opt dump.ll -basicaa -sroa ... -S -o optdump.ll

to then reproduce the error with

    opt optdump.ll -polly-opt-isl -polly-codegen -analyze

However, the IR is not the same. -O3 uses a PassBuilder than creates passes
with different parameters than the default.

Dumping the IR after Polly is useful to compare a miscompilation with
a known-good configuration.

Differential Revision: https://reviews.llvm.org/D30788

llvm-svn: 297415