Backport changes for emscripten 3.1.40 to LLVM 16

[willcohen]

The following two commits on LLVM 17 are needed for the latest release of emscripten to build with LLVM 16 (see emscripten-core/emscripten#19466). That release requires the functionality of https://reviews.llvm.org/D145308, and that commit appears to depend on https://reviews.llvm.org/D145431 as well.
/cherry-pick 8aef04f 3111784

[willcohen]

(I'm not totally clear on how automated this backport process is for new contributors, but I can confirm that the two commits apply cleanly for me to the 16 branch as is.)

[llvmbot]

The following two commits on LLVM 17 are needed for the latest release of emscripten to build with LLVM 16 (see emscripten-core/emscripten#19466). That release requires the functionality of https://reviews.llvm.org/D145308, and that commit appears to depend on https://reviews.llvm.org/D145431 as well.
/cherry-pick 8aef04f 3111784

Error: Command failed due to missing milestone.

[nikic]

/cherry-pick 8aef04f 3111784

[llvmbot]

/branch llvm/llvm-project-release-prs/issue63040

[llvmbot]

/pull-request llvm/llvm-project-release-prs#462