[cmake] Enable building clad within the LLVM monorepo.

[vgvassilev]

This patch allows us to build with `cmake -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_EXTERNAL_PROJECTS=clad -DLLVM_EXTERNAL_CLAD_SOURCE_DIR=<path_to_clad> ...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.56%. Comparing base (fe11e45) to head (03cf3fa).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1157      +/-   ##
==========================================
+ Coverage   94.43%   94.56%   +0.13%     
==========================================
  Files          51       51              
  Lines        8928     8944      +16     
==========================================
+ Hits         8431     8458      +27     
+ Misses        497      486      -11     

see 3 files with indirect coverage changes

see 3 files with indirect coverage changes