[flang] Add Clang CMake modules path for Decimal library

This fixes an error that clang_target_link_libraries is unknown
when building the Decimal library standalone--this is the same
as D149090, just for the Decimal library (vs the runtime).

I was still seeing this error for the Decimal library and didn't
see a corresponding patch--apologies in advance if I missed one
or if this is a duplicate!

Differential Revision: https://reviews.llvm.org/D149277