[clang-format] Fix working -assume-filename with .clang-format-ignore

The filename given by the `-assume-filename` option is used to search
for `.clang-format` files, etc., but is not used to match the contents
of the `.clang-format-ignore` file.

Fixed that when the `-assume-filename` option is specified, the
`.clang-format-ignore` file is processed for that filename.

clang/test/Format/clang-format-ignore.cpp

@@ -46,5 +46,16 @@
 // CHECK5-NEXT: {{Formatting \[4/5] .*foo\.c}}
 // CHECK5-NOT: foo.js
 
+// RUN: echo "foo.*" > .clang-format-ignore
+// RUN: touch foo.c
+// RUN: echo foo | clang-format -assume-filename=foo.c 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=CHECK6 -allow-empty
+// CHECK6-NOT: foo
+
+// RUN: touch bar.c
+// RUN: echo foo | clang-format -assume-filename=bar.c 2>&1 \
+// RUN:   | FileCheck %s -check-prefix=CHECK7 -allow-empty
+// CHECK7: foo
+
 // RUN: cd ..
 // RUN: rm -r %t.dir

clang/tools/clang-format/ClangFormat.cpp

@@ -707,8 +707,11 @@ int main(int argc, const char **argv) {
     errs() << "Clang-formatting " << LineNo << " files\n";
   }
 
-  if (FileNames.empty())
+  if (FileNames.empty()) {
+    if (!AssumeFileName.empty() && isIgnored(AssumeFileName))
+      return 0;
     return clang::format::format("-", FailOnIncompleteFormat);
+  }
 
   if (FileNames.size() > 1 &&
       (!Offsets.empty() || !Lengths.empty() || !LineRanges.empty())) {