Move -Wswitch from -Wmost to -Wall, consitent with GCC.

This is a relatively noisy warning for a codebase not explicitly designed for
it (effectively enforcing a stylistic constraint about the use of defaults
in switches over enums) & there's nothing Clang does to clean up the noise
when compared to GCC's implementation so the same decision seems suitable.

llvm-svn: 150230