Use dedicated module cache to build xctest, llbuild and Foundation (swiftlang#34728)

This is a small step to ensure multiple build jobs running on the same
machine (e.g. in CI) do not stomp on each other.

This PR does not cover the version detection of the Swift compiler
during the CMake configuration -- in there the compiler gets called once
without any of the `CMAKE_Swift_FLAGS`.

Addresses rdar://71373494

Author: edymtt

utils/build-script-impl

@@ -2143,6 +2143,7 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DCMAKE_CXX_COMPILER:PATH="${CLANG_BIN}/clang++"
                     -DCMAKE_INSTALL_PREFIX:PATH="$(get_host_install_prefix ${host})"
                     -DCMAKE_Swift_COMPILER:PATH=${SWIFTC_BIN}
+                    -DCMAKE_Swift_FLAGS:STRING="-module-cache-path \"${module_cache}\""
 
                     -DLLBUILD_ENABLE_ASSERTIONS:BOOL=$(true_false "${LLBUILD_ENABLE_ASSERTIONS}")
                     -DLLBUILD_SUPPORT_BINDINGS:=Swift
@@ -2215,6 +2216,7 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DCMAKE_C_COMPILER:PATH="${CLANG_BIN}/clang"
                     -DCMAKE_CXX_COMPILER:PATH="${CLANG_BIN}/clang++"
                     -DCMAKE_Swift_COMPILER:PATH=${SWIFTC_BIN}
+                    -DCMAKE_Swift_FLAGS:STRING="-module-cache-path \"${module_cache}\""
                     -DCMAKE_INSTALL_PREFIX:PATH="$(get_host_install_prefix ${host})"
                     -DCMAKE_INSTALL_LIBDIR:PATH="lib"
 
@@ -2283,6 +2285,7 @@ for host in "${ALL_HOSTS[@]}"; do
                   -DCMAKE_CXX_COMPILER:PATH=${CLANG_BIN}/clang++
                   -DCMAKE_SWIFT_COMPILER:PATH=${SWIFTC_BIN}
                   -DCMAKE_Swift_COMPILER:PATH=${SWIFTC_BIN}
+                  -DCMAKE_Swift_FLAGS:STRING="-module-cache-path \"${module_cache}\""
                   -DCMAKE_INSTALL_PREFIX:PATH=$(get_host_install_prefix ${host})
 
                   ${LIBICU_BUILD_ARGS[@]}