Backport LLVM's r341717 "Fix flags used to compile benchmark library with clang-cl" (#673)

`MSVC` is true for clang-cl, but `"${CMAKE_CXX_COMPILER_ID}" STREQUAL
"MSVC"` is false, so we would enable -Wall, which means -Weverything
with clang-cl, and we get tons of undesired warnings.

Use the simpler condition to fix things.

Patch by: Reid Kleckner @rnk