Drop the original buffer ID from GeneratedSourceInfo.

Nobody is using it, and this information is recoverable from the original
source range.

Author: DougGregor

include/swift/Basic/SourceManager.h

@@ -41,10 +41,6 @@ class GeneratedSourceInfo {
     PrettyPrinted,
   } kind;
 
-  /// The buffer ID for the enclosing buffer, in which originalSourceRange
-  /// resides.
-  unsigned originalBufferID;
-
   /// The source range in the enclosing buffer where this source was generated.
   ///
   /// This could point at a macro expansion or at the implicit location at

lib/AST/DiagnosticEngine.cpp

@@ -1241,7 +1241,6 @@ DiagnosticEngine::diagnosticInfoForDiagnostic(const Diagnostic &diagnostic) {
             bufferID,
             GeneratedSourceInfo{
               GeneratedSourceInfo::PrettyPrinted,
-              bufferID,
               SourceRange(),
               SourceRange(
                   memBufferStartLoc,

lib/IDETool/CompileInstance.cpp

@@ -193,7 +193,6 @@ void retypeCheckFunctionBody(AbstractFunctionDecl *func,
       sliceBufferID,
       GeneratedSourceInfo{
         GeneratedSourceInfo::ReplacedFunctionBody,
-        *func->getParentSourceFile()->getBufferID(),
         func->getOriginalBodySourceRange(),
         newRange,
         func,

lib/IDETool/IDEInspectionInstance.cpp

@@ -346,7 +346,6 @@ bool IDEInspectionInstance::performCachedOperationIfPossible(
         newBufferID,
         GeneratedSourceInfo{
           GeneratedSourceInfo::ReplacedFunctionBody,
-          *AFD->getParentSourceFile()->getBufferID(),
           AFD->getOriginalBodySourceRange(),
           newBodyRange,
           AFD,

lib/Sema/TypeCheckMacros.cpp

@@ -448,7 +448,6 @@ Expr *swift::expandMacroExpr(
   auto macroBufferRange = sourceMgr.getRangeForBuffer(macroBufferID);
   GeneratedSourceInfo sourceInfo{
     GeneratedSourceInfo::MacroExpansion,
-    *sourceFile->getBufferID(),
     expr->getSourceRange(),
     SourceRange(macroBufferRange.getStart(), macroBufferRange.getEnd()),
     ASTNode(expr).getOpaqueValue(),