Remove compileMutex to enable parallel compilation on Windows (#583)

The issue observed in 76bc7f9 seems fixed on llvm-14 branch based
on new testing of a multi-threaded workload on Ice Lake server and Alder
Lake desktop.
The same workload still fails on llvm-9 branch on Alder Lake.

Author: wenju-he

common_clang.cpp

@@ -85,8 +85,6 @@ static volatile bool lazyCCInit =
     true; // the flag must be 'volatile' to prevent caching in a CPU register
 static llvm::sys::Mutex lazyCCInitMutex;
 
-llvm::ManagedStatic<llvm::sys::SmartMutex<true>> compileMutex;
-
 void CommonClangTerminate() { llvm::llvm_shutdown(); }
 
 // This function mustn't be invoked from a static object constructor,
@@ -212,9 +210,6 @@ Compile(const char *pszProgramSource, const char **pInputHeaders,
   CommonClangInitialize();
 
   try {
-#ifdef _WIN32
-    llvm::sys::SmartScopedLock<true> compileGuard{*compileMutex};
-#endif
     std::unique_ptr<OCLFEBinaryResult> pResult(new OCLFEBinaryResult());
 
     // Create the clang compiler
@@ -226,9 +221,6 @@ Compile(const char *pszProgramSource, const char **pInputHeaders,
     // Prepare error log
     llvm::raw_string_ostream err_ostream(pResult->getLogRef());
     {
-#ifndef _WIN32
-      llvm::sys::SmartScopedLock<true> compileGuard{*compileMutex};
-#endif
       // Parse options
       optionsParser.processOptions(pszOptions, pszOptionsEx);
 
@@ -347,9 +339,6 @@ Compile(const char *pszProgramSource, const char **pInputHeaders,
       err_ostream.flush();
     }
     {
-#ifndef _WIN32
-      llvm::sys::SmartScopedLock<true> compileGuard{*compileMutex};
-#endif
       if (pBinaryResult) {
         *pBinaryResult = pResult.release();
       }

options_compile.cpp

@@ -39,8 +39,6 @@ Copyright (c) Intel Corporation (2009-2017).
 
 using namespace llvm::opt;
 
-extern llvm::ManagedStatic<llvm::sys::SmartMutex<true>> compileMutex;
-
 static const OptTable::Info ClangOptionsInfoTable[] = {
 #define PREFIX(NAME, VALUE)
 #define OPTION(PREFIX, NAME, ID, KIND, GROUP, ALIAS, ALIASARGS, FLAGS, PARAM,  \
@@ -442,10 +440,6 @@ std::string CompileOptionsParser::getEffectiveOptionsAsString() const {
 extern "C" CC_DLL_EXPORT bool CheckCompileOptions(const char *pszOptions,
                                                   char *pszUnknownOptions,
                                                   size_t uiUnknownOptionsSize) {
-  // LLVM doesn't guarantee thread safety,
-  // therefore we serialize execution of LLVM code.
-  llvm::sys::SmartScopedLock<true> compileOptionsGuard{*compileMutex};
-
   try {
     CompileOptionsParser optionsParser("200");
     return optionsParser.checkOptions(pszOptions, pszUnknownOptions,