cmake: use llvm dir variables for clang/utils/hmaptool

Install hmaptool using the LLVM specific variables, so
everything goes in the right place in case llvm is included
from a top level CMakeLists.txt.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: stephenneuendorffer

Differential Revision: https://reviews.llvm.org/D126308