Use the new llvm_canonicalize_cmake_booleans() function to canonicalize
booleans for lit tests. Replace the duplicate ENABLE_CLANG* variables
used to hold canonicalized values with in-place canonicalization. Use
implicit logic in Python code to avoid overrelying on exact 0/1 values.

Differential Revision: https://reviews.llvm.org/D28529

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293052 91177308-0d34-0410-b5e6-96231b3b80d8