build: avoid modifying `CMAKE_CXX_FLAGS`
build: avoid modifying `CMAKE_CXX_FLAGS`

`CMAKE_CXX_FLAGS` is a special variable that should not be modified as
it is cached and the user specified values will take precedence.  Use
`add_compile_options` to specify the desired flags.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild