[test] Port clang tests to canonicalized booleans

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

llvm-svn: 293052