[NFC][clang] Refine tests by adding `:` to checks

[Fznamznon]

Summary

The tests can fail if wokring directory where the tests were launched
has a error substring in its path.

[benlangmuir]

LGTM. The vfsroot-include.c is a -cc1 invocation, so it could probably be switched to use -verify instead of FileCheck'ing the output, unless there's something I'm missing. But that doesn't need to be for this patch.

clang/test/Driver/unknown-std.S

 // RUN: %clang -std=c++11 %s -E -o /dev/null 2>&1 | FileCheck %s --allow-empty
-// CHECK-NOT: error
+// CHECK-NOT: error:

clang/test/Index/allow-editor-placeholders.cpp

 // RUN: c-index-test -test-load-source all %s 2>&1 | FileCheck %s
 
 <#placeholder#>;
 
-// CHECK-NOT: error
+// CHECK-NOT: error:

clang/test/VFS/real-path-found-first.m

 // RUN: c-index-test -index-file %s -fmodules -fimplicit-module-maps -fmodules-cache-path=%t-cache -F %t \
 // RUN:     -ivfsoverlay %t.yaml -fsyntax-only -Rmodule-include-translation \
 // RUN:     -Werror=non-modular-include-in-framework-module | FileCheck %s
 // RUN: echo ' ' >> %t/SomeFramework.framework/Modules/module.modulemap
 // RUN: c-index-test -index-file %s -fmodules -fimplicit-module-maps -fmodules-cache-path=%t-cache -F %t \
 // RUN:     -ivfsoverlay %t.yaml -fsyntax-only -Rmodule-include-translation \
 // RUN:     -Werror=non-modular-include-in-framework-module | FileCheck %s
 // CHECK: warning: treating
-// CHECK-NOT: error
+// CHECK-NOT: error:
 
 // With a VFS-mapped module map file
 // RUN: mv %t/SomeFramework.framework/Modules/module.modulemap %t/hide_module.map
 // RUN: echo "{ 'version': 0, 'roots': [ { " > %t2.yaml
 // RUN: echo "'name': '%t/SomeFramework.framework/Modules/module.modulemap'," >> %t2.yaml
 // RUN: echo "'type': 'file', 'external-contents': '%t/hide_module.map' } ] }" >> %t2.yaml
 
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t-cache -F %t \

clang/test/VFS/vfsroot-include.c

 // RUN: rm -rf %t
 // RUN: mkdir -p %t
 // RUN: sed -e "s@TEST_DIR@%{/S:regex_replacement}@g" -e "s@OUT_DIR@%{/t:regex_replacement}@g" %S/Inputs/vfsroot.yaml > %t.yaml
 // RUN: not %clang_cc1 -Werror -ivfsoverlay %t.yaml -I %S/Inputs -I /direct-vfs-root-files -fsyntax-only /tests/vfsroot-include.c 2>&1 | FileCheck %s
 // The line above tests that the compiler input file is looked up through VFS.
 
 // Test successful include through the VFS.
 #include "not_real.h"
 
 // Test that a file missing from the VFS root is not found, even if it is
 // discoverable through the real file system. Fatal error should be the last
 // in the file as it hides other errors.
 #include "actual_header.h"
 // CHECK: fatal error: 'actual_header.h' file not found
 // CHECK: 1 error generated.
-// CHECK-NOT: error
+// CHECK-NOT: error: