[Caching] Fix a bug when emitting diagnostics from nested macros

Fix a bug when emitting cached diagnostics from nested macros. When
building a new source manager for cached diagnostics, there is a bug
when creating mapping between FileIDs between two different source
managers. If the newly created file in source manager requires another
file to be mapped, e.g. when emitting a diagnostic from nested macros,
the returning FileID can be updated by the second request before
returning. Fix the bug by making sure two different FileIDs are returned
in this case.

rdar://144810862

Author: cachemeifyoucan

lib/Frontend/CachedDiagnostics.cpp

@@ -348,7 +348,7 @@ unsigned DiagnosticSerializer::getFileIDFromBufferID(SourceManager &SM,
 
   Files.emplace_back(std::move(File));
   Allocated.insert({Idx, ++CurrentFileID});
-
+  unsigned NewFileID = CurrentFileID;
 
   auto Info = SM.getGeneratedSourceInfo(Idx);
   auto convertGeneratedFileInfo =
@@ -364,7 +364,7 @@ unsigned DiagnosticSerializer::getFileIDFromBufferID(SourceManager &SM,
     GeneratedFileInfo.emplace_back(std::move(GI));
   }
 
-  return CurrentFileID;
+  return NewFileID;
 }
 
 SerializedSourceLoc

test/CAS/cached_diagnostics_macro.swift

@@ -49,12 +49,23 @@ public struct CallDeprecatedMacro: ExpressionMacro {
   }
 }
 
+public struct ToMyWarningMacro: ExpressionMacro {
+   public static func expansion(
+     of macro: some FreestandingMacroExpansionSyntax,
+     in context: some MacroExpansionContext
+   ) throws -> ExprSyntax {
+     return "#myWarning(\"\")"
+  }
+}
+
 //--- main.swift
 @freestanding(expression) macro myWarning(_ message: String) = #externalMacro(module: "MacroDefinition", type: "CallDeprecatedMacro")
+@freestanding(expression) macro toMyWarning(_ message: String) = #externalMacro(module: "MacroDefinition", type: "ToMyWarningMacro")
 
 @available(*, deprecated)
 func testDeprecated() {}
 
 func testDiscardableStringify(x: Int) {
+  #toMyWarning("this is a warning")
   #myWarning("this is a warning")
 }