Add @import support for Xcode projects

Summary:
@public
This is a workaround to a clang crash that happens whenever `-fmodules` and `YojsonASTExporter` are used together.
This workaround, uses the `-plugin` argument instead of `-add-plugin` one for the clang frontend, and as a result of that, it overrides the default action of clang, which means no object files are emitted, but just the AST.
To generate the missing data needed by the subsequent building phases of xcodebuild, we run Apple's clang.

Test Plan:
Compiled project containing Pods and `@import`, through a command of the form:
  infer -- xcodebuild -workspace project_name.xcworkspace -scheme project_name -sdk iphonesimulator

infer/lib/capture/clang

@@ -16,4 +16,5 @@ if [ -z $INFER_RESULTS_DIR ]; then
     exit $?
 fi
 
+
 "${SCRIPT_PATH%/}/../clang/clang_general_wrapper$XX" "$@"

infer/lib/capture/xcodebuild.py

@@ -40,19 +40,20 @@ class XcodebuildCapture:
 
         # get the path to 'true' using xcrun
         true_path = subprocess.check_output(['xcrun', '--find', 'true']).strip()
+        apple_clang_path = \
+            subprocess.check_output(['xcrun', '--find', 'clang']).strip()
 
         # these settings will instruct xcodebuild on which clang to use
-        # and to not run any linking
-        self.cmd += ['LD={true_path}'.format(true_path=true_path)]
-        self.cmd += ['LDPLUSPLUS={true_path}'.format(true_path=true_path)]
         self.cmd += ['CC={wrapper}'.format(wrapper=CLANG_WRAPPER)]
         self.cmd += ['CPLUSPLUS={wrapper}'.format(wrapper=CLANGPLUSPLUS_WRAPPER)]
-        self.cmd += ['LIPO={true_path}'.format(true_path=true_path)]
 
-        env_vars['INFER_RESULTS_DIR'] = self.args.infer_out
+        # skip the ProcessPCH phase to fix the "newer/older" incompatibility
+        # error for the pch files generated by apple's clang and
+        # the open-source one
+        self.cmd += ['GCC_PRECOMPILE_PREFIX_HEADER=NO']
 
-        # fix the GenerateDSYMFile error
+        env_vars['INFER_RESULTS_DIR'] = self.args.infer_out
-        self.cmd += ["DEBUG_INFORMATION_FORMAT='dwarf'"]
+        env_vars['FCP_APPLE_CLANG'] = apple_clang_path
 
         try:
             subprocess.check_call(self.cmd, env=env_vars)

infer/lib/clang/clang_general_wrapper

@@ -35,6 +35,8 @@ INFER_FRONTEND_ARGS=($FCP_INFER_FRONTEND_ARGS)
 REPORT_FRONTEND_FAILURE="${FCP_REPORT_FRONTEND_FAILURE}"
 # enable debug mode (to get more data saved to disk for future inspections)
 DEBUG_MODE="${FCP_DEBUG_MODE}"
+# specify where is located Apple's clang
+APPLE_CLANG="${FCP_APPLE_CLANG}"
 
 if [ -z "$RESULTS_DIR" ]; then
     echo '$FCP_RESULTS_DIR with the name of the output directory not provided.' > /dev/stderr
@@ -105,11 +107,20 @@ then
 
     if [ -n "$SOURCE_FILE" ]
     then
+        # (t7400979) this is a workaround to avoid that clang crashes when the -fmodules flag
+        # and the YojsonASTExporter plugin are used. Since the -plugin argument disables
+        # the generation of .o files, we invoke apple clang again to generate the expected
+        # artifacts. This will keep xcodebuild plus all the sub-steps happy.
+        if [ -n "$APPLE_CLANG" ]; then
+            ADD_PLUGIN_FLAG="-plugin"
+        else
+            ADD_PLUGIN_FLAG="-add-plugin"
+        fi
         ATTACH_PLUGIN="1"
         IFS=$'\n'
         EXTRA_ARGS=("-Xclang" "-load"
             "-Xclang" "${PLUGIN_PATH}"
-            "-Xclang" "-add-plugin"
+            "-Xclang" "$ADD_PLUGIN_FLAG"
             "-Xclang" "${PLUGIN_NAME}")
         if [ -n "$SYNTAX_ONLY" ]; then
             EXTRA_ARGS+=("-fsyntax-only")
@@ -164,4 +175,9 @@ else
     STATUS=$?
 fi
 
+# run apple clang if required (and if any)
+if [ -n "$APPLE_CLANG" ]; then
+    "${APPLE_CLANG}$XX" "$@" || exit $?
+fi
+
 exit $STATUS

infer/lib/clang/clang_wrapper

@@ -15,7 +15,7 @@ COMMAND=("${CLANG_COMPILER}${XX}")
 # Remove command line options not supported by the opensource compiler or the plugins.
 for X in "$@"
 do
-    if [ "$X" != "-fapplication-extension" ] && [ "$X" != "-fmodules" ]
+    if [ "$X" != "-fapplication-extension" ]
     then
         COMMAND+=("$X")
     fi