[cmake] Modernize some conditionals. NFC

The "x${...}" form was a workaround for CMake versions prior to 3.1,
where the if command would interpret arguments as variables even when
quoted [1]. We can drop the workaround now that our minimum CMake
version is 3.4.

[1] https://cmake.org/cmake/help/v3.1/policy/CMP0054.html

Differential Revision: https://reviews.llvm.org/D40744

llvm-svn: 319723