Add llvm-pdbutil in LLVM_TOOLCHAIN_TOOLS

One can argue that it qualifies as a toolchain tool rather than "internal llvm
tool". This will make it part of builds which set the
LLVM_INSTALL_TOOLCHAIN_ONLY cmake option, such as the Windows installer.

Fixes https://github.com/llvm/llvm-project/issues/53461

Differential revision: https://reviews.llvm.org/D118606