[LinkerWrapper] Forward `-mllvm` options to the linker wrapper

This patch adds the ability to use `-mllvm` options in the linker
wrapper when performing bitcode linking or the module compilation.
This is done by passing in the LLVM argument to the clang-linker-wrapper
tool. Inside the linker-wrapper tool we invoke the `CommandLine` parser
solely for forwarding command line options to the `clang-linker-wrapper`
to the LLVM tools that also use the `CommandLine` parser. The actual
arguments to the linker wrapper are parsed using the `Opt` library
instead.

For example, in the following command the `CommandLine` parser will attempt to
parse `abc`, while the `opt` parser takes `-mllvm <arg>` and ignores it so it is
not passed to the linker arguments.
```
clang-linker-wrapper -mllvm -abc -- <linker-args>
```

As far as I can tell this is the easiest way to forward arguments to
LLVM tool invocations. If there is a better way to pass these arguments
(such as through the LTO config) let me know.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D129424

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