[cmake] Link in the toolchain c++ headers after running LLVM cmake but before the LLVM build.

When we are building LLVM create symlinks to the c++ headers. We need to do this
before building LLVM since compiler-rt depends on being built with the just
built clang compiler. These are normally put into place during the cmake step of
LLVM's build when libcxx is in tree... but we are not building llvm with libcxx
in tree when we build swift. So we need to do cmake's work here.

rdar://26284108

Author: gottesmm

utils/build-script-impl

@@ -1891,6 +1891,31 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_COMPILE_TOOLS_DEPLOYMENT_TARG
             { set +x; } 2>/dev/null
         fi
 
+        # When we are building LLVM create symlinks to the c++ headers. We need
+        # to do this before building LLVM since compiler-rt depends on being
+        # built with the just built clang compiler. These are normally put into
+        # place during the cmake step of LLVM's build when libcxx is in
+        # tree... but we are not building llvm with libcxx in tree when we build
+        # swift. So we need to do configure's work here.
+        if [[ "${product}" == "llvm" ]]; then
+            # Find the location of the c++ header dir.
+            if [[ "$(uname -s)" == "Darwin" ]] ; then
+              HOST_CXX_DIR=$(dirname ${HOST_CXX})
+              HOST_CXX_HEADERS_DIR="$HOST_CXX_DIR/../../usr/include/c++"
+            else # Linux
+              HOST_CXX_HEADERS_DIR="/usr/include/c++"
+            fi
+
+            # Find the path in which the local clang build is expecting to find
+            # the c++ header files.
+            BUILT_CXX_INCLUDE_DIR="$llvm_build_dir/include"
+
+            echo "symlinking the system headers ($HOST_CXX_HEADERS_DIR) into the local clang build directory ($BUILT_CXX_INCLUDE_DIR)."
+            set -x
+            ln -s -f "$HOST_CXX_HEADERS_DIR" "$BUILT_CXX_INCLUDE_DIR"
+            { set +x; } 2>/dev/null
+        fi
+
         # Build.
         if [[ ! "${skip_build}" ]]; then
             if [[ "${CMAKE_GENERATOR}" == "Xcode" ]] ; then
@@ -1910,26 +1935,6 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_COMPILE_TOOLS_DEPLOYMENT_TARG
             ${DISTCC_PUMP} "${CMAKE}" --build "${build_dir}" $(cmake_config_opt ${product}) -- "${BUILD_ARGS[@]}" ${build_targets[@]}
             { set +x; } 2>/dev/null
         fi
-
-        # When we are building LLVM create symlinks to the c++ headers.
-        if [[ "${product}" == "llvm" ]]; then
-            # Find the location of the c++ header dir.
-            if [[ "$(uname -s)" == "Darwin" ]] ; then
-              HOST_CXX_DIR=$(dirname ${HOST_CXX})
-              HOST_CXX_HEADERS_DIR="$HOST_CXX_DIR/../../usr/include/c++"
-            else # Linux
-              HOST_CXX_HEADERS_DIR="/usr/include/c++"
-            fi
-
-            # Find the path in which the local clang build is expecting to find
-            # the c++ header files.
-            BUILT_CXX_INCLUDE_DIR="$llvm_build_dir/include"
-
-            echo "symlinking the system headers ($HOST_CXX_HEADERS_DIR) into the local clang build directory ($BUILT_CXX_INCLUDE_DIR)."
-            set -x
-            ln -s -f "$HOST_CXX_HEADERS_DIR" "$BUILT_CXX_INCLUDE_DIR"
-            { set +x; } 2>/dev/null
-        fi
     done
 done
 # END OF BUILD PHASE