Suport passing arbitrary llvm arguments (#271)

- Support for passing LLVM command line options via the prcoess input or
providing one or more `--llvm-arg='..'` resolc CLI flag. This allows
more fine-grained control over the LLVM backend configuration.
- Make LLVM initialization idempotent.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/solidity/src/resolc/arguments.rs

@@ -166,6 +166,10 @@ pub struct Arguments {
     #[cfg(debug_assertions)]
     #[arg(long = "recursive-process-input")]
     pub recursive_process_input: Option<String>,
+
+    #[arg(long = "llvm-arg")]
+    /// These are passed to LLVM as the command line to allow manual control.
+    pub llvm_arguments: Vec<String>,
 }
 
 impl Arguments {