test: Only warn about missing substitutions for required tools

Every time lit is invoked, I get warnings like so:

  lit.py: lit.cfg:286: note: Did not find llvm-go in /Users/bogner/build/llvm/./bin
  lit.py: lit.cfg:286: note: Did not find Kaleidoscope-Ch3 in /Users/bogner/build/llvm/./bin

Since these tools are only built in certain configs, these warnings
are superfluous. Change it so that we only warn about tools that are
built in all configs.

llvm-svn: 246684