CMake allows you to set a custom CXX compiler for the API test suite.
However, this variable is never used, because dotest uses the same
compiler to build C and CXX sources.
I'm not sure if this variable was added with the intention of supporting
a different compiler or if this is just a remnant of old functionality.
Given that this hasn't been working for a while, I assume it's safe to
remove.
Differential revision: https://reviews.llvm.org/D69401
d52b36e354