[Safe Buffers] Serialize unsafe_buffer_usage pragmas (#92031)

The commit adds serialization and de-serialization implementations for
the stored regions. Basically, the serialized representation of the
regions of a PP is a (ordered) sequence of source location encodings.
For de-serialization, regions from loaded files are stored by their ASTs.
When later one queries if a loaded location L is in an opt-out
region, PP looks up the regions of the loaded AST where L is at.

(Background if helps: a pair of `#pragma clang unsafe_buffer_usage begin/end` pragmas marks a
warning-opt-out region. The begin and end locations (opt-out regions)
are stored in preprocessor instances (PP) and will be queried by the
`-Wunsafe-buffer-usage` analyzer.)

The reported issue at upstream: https://github.com/llvm/llvm-project/issues/90501
rdar://124035402

1 files changed, 18 insertions, 0 deletions

diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
index 753601e..f0af1a3 100644
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -1915,6 +1915,24 @@ SourceManager::getDecomposedIncludedLoc(FileID FID) const {
   return DecompLoc;
 }
 
+FileID SourceManager::getUniqueLoadedASTFileID(SourceLocation Loc) const {
+  assert(isLoadedSourceLocation(Loc) &&
+         "Must be a source location in a loaded PCH/Module file");
+
+  auto [FID, Ignore] = getDecomposedLoc(Loc);
+  // `LoadedSLocEntryAllocBegin` stores the sorted lowest FID of each loaded
+  // allocation. Later allocations have lower FileIDs. The call below is to find
+  // the lowest FID of a loaded allocation from any FID in the same allocation.
+  // The lowest FID is used to identify a loaded allocation.
+  const FileID *FirstFID =
+      llvm::lower_bound(LoadedSLocEntryAllocBegin, FID, std::greater<FileID>{});
+
+  assert(FirstFID &&
+         "The failure to find the first FileID of a "
+         "loaded AST from a loaded source location was unexpected.");
+  return *FirstFID;
+}
+
 bool SourceManager::isInTheSameTranslationUnitImpl(
     const std::pair<FileID, unsigned> &LOffs,
     const std::pair<FileID, unsigned> &ROffs) const {