LLVM 18.1.2

[h-vetinari]

Blockers for merging this PR and thus enabling the compilers in conda-forge (indentation denotes dependency; c.f. list from 17.x):

• llvmdev v18.1.2 llvmdev-feedstock#258
  • clangdev v18.1.2 clangdev-feedstock#276
    • Build for LLVM 18 cctools-and-ld64-feedstock#65
    • compiler-rt v18.1.2 compiler-rt-feedstock#103
      • openmp v18.1.2 openmp-feedstock#119
    • libcxx 17.0.6 libcxx-feedstock#131 (waiting for MacOS 10.13 roll-out)
  • lld v18.1.2 lld-feedstock#88

Related feedstocks for LLVM 18 support more generally:

• Add clang 18 clang-win-activation-feedstock#33 (windows side of this PR; needs this PR)
• Add clang 18 ctng-compiler-activation-feedstock#104 (linux side of this PR; needs this PR)
• flang v18.1.2 flang-feedstock#44 (needs mlir, openmp, compiler-rt)
• Add testing against clang 17 & 18 libcxx-testing-feedstock#6 (needs this PR)
• lldb v18.1.2 lldb-feedstock#62 (needs this PR)
• mlir v18.1.2 mlir-feedstock#65 (needs llvmdev)
• mlir-python-bindings v18.1.3 mlir-python-bindings-feedstock#23 (needs llvmdev & mlir)

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

@conda-forge-admin, pleaae rerender

[h-vetinari]

@conda-forge/clang-compiler-activation this is ready for review, PTAL! :)

[xhochy]

Changes look good but I'm not a maintainer here

[h-vetinari]

OK, since there are no substantial (much less any controversial) changes in LLVM 18 vs. 17, I'm going ahead with merging this one.