Auto merge of #116555 - paulmenage:llvm-module-flag, r=wesleywiser

Add -Z llvm_module_flag

Allow adding values to the `!llvm.module.flags` metadata for a generated module.  The syntax is

`-Z llvm_module_flag=<name>:<type>:<value>:<behavior>`

Currently only u32 values are supported but the type is required to be specified for forward compatibility.  The `behavior` element must match one of the named LLVM metadata behaviors.viors.

This flag is expected to be perma-unstable.

Author: bors