fix check-clang-tools tests that fail due to Windows CRLF line endings

Running check-clang-tools on Windows produces 5 test failures:

Failed Tests (5):
  Clang Tools :: clang-apply-replacements/ClangRenameClassReplacements.cpp
  Clang Tools :: clang-apply-replacements/basic.cpp
  Clang Tools :: clang-apply-replacements/format.cpp
  Clang Tools :: clang-move/move-used-helper-decls.cpp
  Clang Tools :: clang-tidy/infrastructure/export-diagnostics.cpp

Four of these failures are simply due to fixed character position
offsets differing on Windows versus Linux, since Windows line endings
take up two characters instead of one:

clang-apply-replacements/ClangRenameClassReplacements.cpp runs clang-rename -offset=254
clang-apply-replacements/Inputs/basic/file[12].yaml specify e.g.  FileOffset: 148 and Offset: 298
clang-apply-replacements/Inputs/format/{no,yes}.yaml specify e.g.  FileOffset: 94 and Offset: 94
clang-tidy/infrastructure/export-diagnostics.cpp specifies e.g.  CHECK-YAML-NEXT: FileOffset: 30

(The move-used-helper-decls.cpp failure seems more complex; clang-move
adds a blank line after void HelperFun1() {} when
clang-move/Inputs/helper_decls_test.cpp has LF line endings, but does
not add a blank line when the input files has CRLF line endings. That
difference in behavior seems like it may be an actual bug, but I have
yet to track it down.)

Differential Revision: https://reviews.llvm.org/D97625

test/.gitattributes

@@ -0,0 +1,19 @@
+# CRLF (Windows) line endings take two bytes instead of one, so any tests that
+# rely on or check fixed character -offset, Offset: or FileOffset: locations
+# will fail when run on input files checked out with different line endings.
+
+# Most test input files should use native line endings, to ensure that we run
+# tests against both line ending types.
+* text=auto
+
+# These test input files rely on one-byte Unix (LF) line-endings, as they use
+# fixed -offset, FileOffset:, or Offset: numbers in their tests.
+clang-apply-replacements/ClangRenameClassReplacements.cpp text eol=lf
+clang-apply-replacements/Inputs/basic/basic.h text eol=lf
+clang-apply-replacements/Inputs/format/no.cpp text eol=lf
+clang-apply-replacements/Inputs/format/yes.cpp text eol=lf
+clang-tidy/infrastructure/export-diagnostics.cpp text eol=lf
+
+# These test input files rely on two-byte Windows (CRLF) line endings.
+clang-apply-replacements/Inputs/crlf/crlf.cpp text eol=crlf
+clang-apply-replacements/Inputs/crlf/crlf.cpp.expected text eol=crlf