[flang] [clang] [flang][driver] Add support for -isysroot in the frontend (PR #77365)

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``````````bash
darker --check --diff -r 0e4a38018a7228d93d72a31d9fae6855f866dded...797e906a23524332f951d20fa0dd6d12b52705c2 flang/test/lit.cfg.py
``````````

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``````````diff
--- lit.cfg.py	2024-01-09 20:22:57.000000 +0000
+++ lit.cfg.py	2024-01-09 20:26:04.907598 +0000
@@ -124,13 +124,16 @@
     isysroot_flag = ["-isysroot", config.osx_sysroot]
 
 # For each occurrence of a flang tool name, replace it with the full path to
 # the build directory holding that tool.
 tools = [
-    ToolSubst("%flang", command=FindTool("flang-new"),
-              extra_args=isysroot_flag,
-              unresolved="fatal"),
+    ToolSubst(
+        "%flang",
+        command=FindTool("flang-new"),
+        extra_args=isysroot_flag,
+        unresolved="fatal",
+    ),
     ToolSubst(
         "%flang_fc1",
         command=FindTool("flang-new"),
         extra_args=["-fc1"],
         unresolved="fatal",
@@ -168,13 +171,15 @@
         os.path.isfile(libruntime)
         and os.path.isfile(libdecimal)
         and os.path.isdir(include)
     ):
         config.available_features.add("c-compiler")
-        tools.append(ToolSubst("%cc", command=config.cc,
-                               extra_args=isysroot_flag,
-                               unresolved="fatal"))
+        tools.append(
+            ToolSubst(
+                "%cc", command=config.cc, extra_args=isysroot_flag, unresolved="fatal"
+            )
+        )
         tools.append(ToolSubst("%libruntime", command=libruntime, unresolved="fatal"))
         tools.append(ToolSubst("%libdecimal", command=libdecimal, unresolved="fatal"))
         tools.append(ToolSubst("%include", command=include, unresolved="fatal"))
 
 # Add all the tools and their substitutions (if applicable). Use the search paths provided for

``````````

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https://github.com/llvm/llvm-project/pull/77365