[CMake] Don't include LLDB_TEST_COMPILER in cached variable

Summary:
CMake has no builtin mechanism for cache invalidation. As a general convention you want to not expand user-specified variables in other cached variables because they will not get updated when the user changes their specified value.

This patch moves the "-C" option for dotest.py into the LLDB_TEST_COMMON_ARGS and out of the CMake cache. In order to prevent issues with out-of-date cache files on builders I've added code to scrub "-C ${LLDB_TEST_COMPILER}" out of the CMake caches, by Force writing the variable. This code can be removed in a few days once the change has trickled through CI systems.

Reviewers: tfiala, labath, zturner

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D25751

llvm-svn: 284551