[build] Add a new --cross-compile-build-swift-tools flag to disable cross-compiling the compiler (#38441)

This is useful when building cross-compilation toolchains where you want
the stdlib and corelibs cross-compiled but don't want the Swift compiler
cross-compiled too with `--cross-compile-hosts`.

Author: finagolfin

utils/build-script-impl

@@ -292,6 +292,7 @@ KNOWN_SETTINGS=(
     cross-compile-install-prefixes                ""                "semicolon-separated list of install prefixes to use for the cross-compiled hosts. The list expands, so if there are more cross-compile hosts than prefixes, unmatched hosts use the last prefix in the list"
     cross-compile-deps-path                       ""                "path for CMake to look for cross-compiled library dependencies, such as libXML2"
     cross-compile-append-host-target-to-destdir   "1"               "turns on appending the host target name of each cross-compiled toolchain to its install-destdir, to keep them separate from the natively-built toolchain"
+    cross-compile-build-swift-tools               "1"               "set to 1 to cross-compile the Swift host tools, like the Swift compiler"
     skip-merge-lipo-cross-compile-tools           ""                "set to skip running merge-lipo after installing cross-compiled host Swift tools"
     coverage-db                                   ""                "If set, coverage database to use when prioritizing testing"
     skip-local-host-install                       ""                "If we are cross-compiling multiple targets, skip an install pass locally if the hosts match"
@@ -1707,6 +1708,12 @@ for host in "${ALL_HOSTS[@]}"; do
                         "${cmake_options[@]}"
                         -DLLVM_TABLEGEN=$(build_directory "${LOCAL_HOST}" llvm)/bin/llvm-tblgen
                         -DSWIFT_INCLUDE_TEST_BINARIES:BOOL=FALSE
+                        -DSWIFT_INCLUDE_TOOLS:BOOL=$(true_false "${CROSS_COMPILE_BUILD_SWIFT_TOOLS}")
+                    )
+                else
+                    cmake_options=(
+                        "${cmake_options[@]}"
+                        -DSWIFT_INCLUDE_TOOLS:BOOL=$(true_false "${BUILD_SWIFT_TOOLS}")
                     )
                 fi
 
@@ -1779,7 +1786,6 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DSWIFT_NATIVE_LLVM_TOOLS_PATH:STRING="${native_llvm_tools_path}"
                     -DSWIFT_NATIVE_CLANG_TOOLS_PATH:STRING="${native_clang_tools_path}"
                     -DSWIFT_NATIVE_SWIFT_TOOLS_PATH:STRING="${native_swift_tools_path}"
-                    -DSWIFT_INCLUDE_TOOLS:BOOL=$(true_false "${BUILD_SWIFT_TOOLS}")
                     -DSWIFT_BUILD_CLANG_OVERLAYS:BOOL=$(true_false "${BUILD_SWIFT_CLANG_OVERLAYS}")
                     -DSWIFT_BUILD_REMOTE_MIRROR:BOOL=$(true_false "${BUILD_SWIFT_REMOTE_MIRROR}")
                     -DSWIFT_STDLIB_SIL_DEBUGGING:BOOL=$(true_false "${BUILD_SIL_DEBUGGING_STDLIB}")