[clang] Load -fbasic-block-sections=list= through the VFS (llvm#160785)

This PR loads the path from `-fbasic-block-sections=list=<path>` through
the VFS rather than going straight to the real file system. This matches
the behavior of other input files of the compiler.

Author: jansvoboda11

clang/lib/CodeGen/BackendUtil.cpp

@@ -447,7 +447,8 @@ static bool initTargetOptions(const CompilerInstance &CI,
 
   if (Options.BBSections == llvm::BasicBlockSection::List) {
     ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
-        MemoryBuffer::getFile(CodeGenOpts.BBSections.substr(5));
+        CI.getVirtualFileSystem().getBufferForFile(
+            CodeGenOpts.BBSections.substr(5));
     if (!MBOrErr) {
       Diags.Report(diag::err_fe_unable_to_load_basic_block_sections_file)
           << MBOrErr.getError().message();