[Flang][Windows] Disable PCH on Windows for flangFrontend

This patch fixes PCH staleness errors (fatal error: ... mtime changed) for
flangFrontend during incremental Windows builds.

The error occurs because the automatic CMake re-run during incremental
builds updates the timestamp of the PCH source file (cmake_pch.cxx)
relative to the existing PCH artifact (.pch). This happens even without
direct modifications to the headers listed for precompilation.

Apparently, flangFrontend is the only known LLVM target which uses
target_precompile_headers, and this mechanism fails to reliably force a
PCH rebuild after the CMake re-run in this Windows environment.

Author: omjavaid

flang/lib/Frontend/CMakeLists.txt

@@ -73,10 +73,16 @@ add_flang_library(flangFrontend
   clangDriver
 )
 
-target_precompile_headers(flangFrontend PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/dump-parse-tree.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/Bridge.h"]]
-)
+# Precompiled Headers for flangFrontend
+# Only enable PCH on non-Windows platforms, as the current usage of
+# target_precompile_headers appears fragile during incremental builds
+# involving CMake regeneration specifically on Windows.
+if(NOT WIN32)
+  target_precompile_headers(flangFrontend PRIVATE
+    [["flang/Parser/parsing.h"]]
+    [["flang/Parser/parse-tree.h"]]
+    [["flang/Parser/dump-parse-tree.h"]]
+    [["flang/Lower/PFTBuilder.h"]]
+    [["flang/Lower/Bridge.h"]]
+  )
+endif()