[clang-cl, PCH] Support for /Yc and /Yu without filename and #pragma hdrstop

With clang-cl, when the user specifies /Yc or /Yu without a filename
the compiler uses a #pragma hdrstop in the main source file to
determine the end of the PCH. If a header is specified with /Yc or
/Yu #pragma hdrstop has no effect.

The optional #pragma hdrstop filename argument is not yet supported.

Differential Revision: https://reviews.llvm.org/D51391

llvm-svn: 341963
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index b526fd1..b3eb9d5 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -87,6 +87,11 @@
 Windows Support
 ---------------
 
+- clang-cl now supports the use of the precompiled header options /Yc and /Yu
+  without the filename argument. When these options are used without the
+  filename, a `#pragma hdrstop` inside the source marks the end of the
+  precompiled code.
+
 - ...