[CMake] Remove LLDB_TEST_USE_CUSTOM_C(XX)_COMPILER

Given that LLDB_TEST_USE_CUSTOM_C_COMPILER and LLDB_TEST_C_COMPILER are
both set at configuration time, I don't really see the point of having
both. This patch simplifies things and uses the custom C/C++ compiler
when the variable is set, and uses the default one when it's not set.
The variable can be unset by passing -ULLDB_TEST_C_COMPILER to CMake.

Differential revision: https://reviews.llvm.org/D66429

llvm-svn: 369435