rustbuild: Fix MSBuild location of `llvm-config.exe`

For LLD integration the path to `llvm-config` needed to change to inside the
build directory itself (for whatever reason) but the build directory is
different on MSBuild than it is on `ninja` for MSVC builds, so the path to
`llvm-config.exe` was actually wrong and not working!

This commit removes the `Build::llvm_config` function in favor of the source of
truth, the `Llvm` build step itself. The build step was then updated to find the
right build directory for MSBuild as well as `ninja` for where `llvm-config.exe`
is located.

Closes #48749

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