Add support for distributed ThinLTO options to `wasm-ld` driver

[trybka]

The Wasm Driver for LLD seems to be missing options related to distributed ThinLTO.

https://gist.github.com/MaskRay/24f4e2eed208b9d8b0a3752575a665d4#distributed-thinlto may be a good reference.

We are looking to add ThinLTO support to Bazel / Emscripten. wasm-ld appears to be missing flags related specifically to distributed ThinLTO. These flags appear to be used for e.g. mapping index files to objects.

See https://reviews.llvm.org/D46034 for an example of some options added to the ELF driver.

Similarly, https://reviews.llvm.org/D64461 and https://reviews.llvm.org/D64542 have implementations for COFF.

The set of flags that we expect are:

--thinlto-index-only=
--thinlto-emit-imports-files
--thinlto-prefix-replace=

Also missing but not sure if related:

--obj-path=

+cc: @sbc100 @walkingeyerobot @googlewalt

[llvmbot]

@llvm/issue-subscribers-lld-wasm

Author: Tom Rybka (trybka)

The Wasm Driver for LLD seems to be missing options related to distributed ThinLTO. https://gist.github.com/MaskRay/24f4e2eed208b9d8b0a3752575a665d4#distributed-thinlto may be a good reference. We are looking to add ThinLTO support to Bazel / Emscripten. `wasm-ld` appears to be missing flags related specifically to _distributed_ ThinLTO. These flags appear to be used for e.g. mapping index files to objects. See https://reviews.llvm.org/D46034 for an example of some options added to the ELF driver. Similarly, https://reviews.llvm.org/D64461 and https://reviews.llvm.org/D64542 have implementations for COFF. The set of flags that we expect are: ``` --thinlto-index-only= --thinlto-emit-imports-files --thinlto-prefix-replace= ``` Also missing but not sure if related: ``` --obj-path= ``` +cc: @sbc100 @walkingeyerobot @googlewalt