Lex: Fix bug in __BASE_FILE__ implementation.

[pcc]

Summary

BASE_FILE previously expanded to "<built-in>" instead of the name
of the main source file in files included using -include.

Also added test coverage for BASE_FILE since there doesn't seem
to be any.

clang/lib/Lex/PPMacroExpansion.cpp

if (II == Ident__LINE__) {
     // Skip down through expansion points until we find a file loc for the
     // end of the expansion history.
     Loc = SourceMgr.getExpansionRange(Loc).getEnd();
     PresumedLoc PLoc = SourceMgr.getPresumedLoc(Loc);
 
     // __LINE__ expands to a simple numeric value.
     OS << (PLoc.isValid()? PLoc.getLine() : 1);
     Tok.setKind(tok::numeric_constant);
-  } else if (II == Ident__FILE__ || II == Ident__BASE_FILE__) {
+  } else if (II == Ident__FILE__) {
     // C99 6.10.8: "__FILE__: The presumed name of the current source file (a
     // character string literal)". This can be affected by #line.
     PresumedLoc PLoc = SourceMgr.getPresumedLoc(Tok.getLocation());
 
-    // __BASE_FILE__ is a GNU extension that returns the top of the presumed
-    // #include stack instead of the current file.
-    if (II == Ident__BASE_FILE__ && PLoc.isValid()) {
-      SourceLocation NextLoc = PLoc.getIncludeLoc();
-      while (NextLoc.isValid()) {
-        PLoc = SourceMgr.getPresumedLoc(NextLoc);
-        if (PLoc.isInvalid())
-          break;
-
-        NextLoc = PLoc.getIncludeLoc();
-      }
-    }
-
     // Escape this filename.  Turn '\' -> '\\' '"' -> '\"'
     SmallString<128> FN;
     if (PLoc.isValid()) {
       FN += PLoc.getFilename();
       Lexer::Stringify(FN);
       OS << '"' << FN << '"';
     }
     Tok.setKind(tok::string_literal);
+  } else if (II == Ident__BASE_FILE__) {
+    SmallString<128> FN =
+        SourceMgr.getFileEntryForID(SourceMgr.getMainFileID())->getName();
+    Lexer::Stringify(FN);
+    OS << '"' << FN << '"';
+    Tok.setKind(tok::string_literal);
   } else if (II == Ident__DATE__) {
     Diag(Tok.getLocation(), diag::warn_pp_date_time);
     if (!DATELoc.isValid())
       ComputeDATE_TIME(DATELoc, TIMELoc, *this);
     Tok.setKind(tok::string_literal);
     Tok.setLength(strlen("\"Mmm dd yyyy\""));
     Tok.setLocation(SourceMgr.createExpansionLoc(DATELoc, Tok.getLocation(),
                                                  Tok.getLocation(),

clang/test/Preprocessor/Inputs/base_file.h

+__BASE_FILE__

clang/test/Preprocessor/base_file.c

+// RUN: %clang_cc1 -include %S/Inputs/base_file.h -E %s | FileCheck %s
+
+// CHECK: {{[\\/]}}base_file.c"
+
+// CHECK: {{[\\/]}}base_file.c"
+#include "Inputs/base_file.h"
+
+// CHECK: {{[\\/]}}base_file.c"
+__BASE_FILE__