[Basic] Fix a warning

This patch fixes:

  clang/lib/Basic/SourceManager.cpp:1979:64: error: 'greater' may not
  intend to support class template argument deduction
  [-Werror,-Wctad-maybe-unsupported]

Author: kazutakahirata

clang/lib/Basic/SourceManager.cpp

@@ -1976,7 +1976,8 @@ bool SourceManager::isInTheSameTranslationUnitImpl(
     auto FindSLocEntryAlloc = [this](FileID FID) {
       // Loaded FileIDs are negative, we store the lowest FileID from each
       // allocation, later allocations have lower FileIDs.
-      return llvm::lower_bound(LoadedSLocEntryAllocBegin, FID, std::greater{});
+      return llvm::lower_bound(LoadedSLocEntryAllocBegin, FID,
+                               std::greater<FileID>{});
     };
 
     // If both are loaded from different AST files.