[docs] Update docs for cmake options LLDB_TEST_C_COMPILER and LLDB_TEST_CXX_COMPILER

Summary:
LLDB_TEST_COMPILER is not a valid option for CMake for LLDB. There are instead two properties LLDB_TEST_C_COMPILER and LLDB_TEST_CXX_COMPILER. Update the documents accordingly to reflect the correct information.


Reviewers: zturner, lldb-commits

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43061

llvm-svn: 324564