[cmake] Strip possibly-inherited compiler flags in in-tree build only

Strip the set of flags (including debug defs, -m32) that could
be inherited from top-level LLVM build only when in-tree build is
performed. This prevents libcxx from confusingly and undesiredly
stripping user-supplied flags e.g. when performing packaging system
controlled multi-ABI build.

Otherwise, in order to perform 32-bit builds the build scripts would
have to use LIBCXX_BUILD_32_BITS. However, -m32 is only one of the many
different ABI flags for different targets, and it really makes no sense
to add separate CMake options for each possible -m* flag and then keep
a mapping from well-known flags to the custom CMake options.

Differential Revision: https://reviews.llvm.org/D24809

llvm-svn: 282475