[SYCL RTC] Introduce --persistent-auto-pch support

clang/include/clang/Driver/Options.td

@@ -7557,6 +7557,10 @@ let Visibility = [SYCLRTCOnlyOption] in {
   let Group = sycl_rtc_only_Group in {
     def auto_pch : Flag<["--"], "auto-pch">,
                    HelpText<"Enable Auto-PCH for SYCL RTC Compilation">;
+    def persistent_auto_pch_EQ
+        : Joined<["--"], "persistent-auto-pch=">,
+          HelpText<"Use Persistent Auto-PCH cache located at <dir> for SYCL "
+                   "RTC Compilation">;
   } // let Group = sycl_rtc_only_Group
 } // let Visibility = [SYCLRTCOnlyOption]
 

clang/include/clang/Frontend/PrecompiledPreamble.h

@@ -132,6 +132,8 @@ class PrecompiledPreamble {
                         IntrusiveRefCntPtr<llvm::vfs::FileSystem> &VFS,
                         llvm::MemoryBuffer *MainFileBuffer) const;
 
+  llvm::StringRef memoryContents() const;
+
 private:
   PrecompiledPreamble(std::unique_ptr<PCHStorage> Storage,
                       std::vector<char> PreambleBytes,

clang/lib/Frontend/PrecompiledPreamble.cpp

@@ -727,6 +727,10 @@ void PrecompiledPreamble::OverridePreamble(
   configurePreamble(Bounds, CI, VFS, MainFileBuffer);
 }
 
+llvm::StringRef PrecompiledPreamble::memoryContents() const {
+  return Storage->memoryContents();
+}
+
 PrecompiledPreamble::PrecompiledPreamble(
     std::unique_ptr<PCHStorage> Storage, std::vector<char> PreambleBytes,
     bool PreambleEndsAtStartOfLine,