Tweak pass management and add some more options

The only changes to the default passes is that O1 now doesn't run the inline
pass, just always-inline with lifetime intrinsics. O2 also now has a threshold
of 225 instead of 275. Otherwise the default passes being run is the same.

I've also added a few more options for configuring the pass pipeline. Namely you
can now specify arguments to LLVM directly via the `--llvm-args` command line
option which operates similarly to `--passes`. I also added the ability to turn
off pre-population of the pass manager in case you want to run *only* your own
passes.

I would consider this as closing #8890. I don't think that we should change the default inlining threshold because LLVM/clang will probably have chosen those numbers more carefully than we would. Regardless, here's the performance numbers from this commit:

```
$ ./x86_64-apple-darwin/stage0/bin/rustc ./gistfile1.rs --test --opt-level=3 -o before
warning: no debug symbols in executable (-arch x86_64)
$ ./before --bench

running 1 test
test bench::aes_bench_x8 ... bench: 1602 ns/iter (+/- 66) = 7990 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

$ ./x86_64-apple-darwin/stage1/bin/rustc ./gistfile1.rs --test --opt-level=3 -o after
warning: no debug symbols in executable (-arch x86_64)
$ ./after --bench

running 1 test
test bench::aes_bench_x8 ... bench: 2103 ns/iter (+/- 175) = 6086 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

$ ./x86_64-apple-darwin/stage1/bin/rustc ./gistfile1.rs --test --opt-level=3 -o after --llvm-args '-inline-threshold=225'
warning: no debug symbols in executable (-arch x86_64)
$ ./after --bench

running 1 test
test bench::aes_bench_x8 ... bench: 1600 ns/iter (+/- 71) = 8000 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

```