cmake: Fix unexpected CMake variable expansion for "WASI"

CMake 3.31 introduced a new variable `WASI` that conflicts with the
sdk name in `SWIFT_SDKS` CMake variable in our build system. With CMake
3.31 and later, tests of `--build-wasm-stdlib` fail because
`if(${SDK} STREQUAL "WASI")` always evaluates to false.
https://cmake.org/cmake/help/v3.31/variable/WASI.html

Author: kateinoigakukun

test/CMakeLists.txt

@@ -407,7 +407,7 @@ foreach(SDK ${SWIFT_SDKS})
       # Reset these values for WASI, as Embedded Swift for WASI requires separate lit configs.
       set(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB_old ${SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB})
       set(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB_CROSS_COMPILING_old ${SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB_CROSS_COMPILING})
-      if(${SDK} STREQUAL "WASI")
+      if("${SDK}" STREQUAL "WASI")
         set(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB OFF)
         set(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB_CROSS_COMPILING OFF)
       endif()