[CMake] Don't include LLDB_TEST_COMPILER in cached variable

[beanz]

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 In order to prevent issues with out-of-date cache files on builders I've added code to scrub "-C ${LLDB_TEST_COMPILER}" out of the CMake caches, by Force writing the variable.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.

lldb/trunk/test/CMakeLists.txt

if( LLDB_DEFAULT_TEST_ARCH STREQUAL "host" )
   string(REGEX MATCH "^[^-]*" LLDB_DEFAULT_TEST_ARCH ${LLVM_HOST_TRIPLE})
 endif ()
 
 # Allow the user to override the default by setting LLDB_TEST_ARCH
 set(LLDB_TEST_ARCH
   ${LLDB_DEFAULT_TEST_ARCH}
   CACHE STRING "Specify the architecture to run LLDB tests as (x86|x64).  Determines whether tests are compiled with -m32 or -m64")
 
+# Scrub LLDB_TEST_COMPILER out of the CMake caches
+# TODO: remove the replace lines and the FORCE parameter in a few days once the
+# change has made its way through bots to clean up their CMake caches.
+string(REPLACE "-C;${LLDB_TEST_COMPILER}" "" LLDB_TEST_USER_ARGS_New "${LLDB_TEST_USER_ARGS}")
+
 # Users can override LLDB_TEST_USER_ARGS to specify arbitrary arguments to pass to the script
 set(LLDB_TEST_USER_ARGS
-  -C ${LLDB_TEST_COMPILER}
-  CACHE STRING "Specify additional arguments to pass to test runner. For example: '-C gcc -C clang -A i386 -A x86_64'")
+  "${LLDB_TEST_USER_ARGS_New}"
+  CACHE STRING "Specify additional arguments to pass to test runner. For example: '-C gcc -C clang -A i386 -A x86_64'" FORCE)
 
 set(LLDB_TEST_COMMON_ARGS
   --arch=${LLDB_TEST_ARCH}
   --executable $<TARGET_FILE:lldb>
   -s
   ${CMAKE_BINARY_DIR}/lldb-test-traces
   -S nm
   -u CXXFLAGS
   -u CFLAGS
+  -C ${LLDB_TEST_COMPILER}
   )
 
 if ( CMAKE_SYSTEM_NAME MATCHES "Windows" )
   # All tests are currently flaky on Windows, so rerun them all once when they fail.
   set(LLDB_TEST_COMMON_ARGS ${LLDB_TEST_COMMON_ARGS} --rerun-all-issues)
   
   set(LLDB_TEST_DEBUG_TEST_CRASHES
     0