[Docs] [HLSL] Add note about PCH support

[python3kgae]

Summary

PCH supported for HLSL is added when compile in -cc1 mode using -include-pch for test AST.
This change add some notes about the support of PCH for HLSL.

[beanz]

clang/docs/HLSL/HLSLSupport.rst

98

We can probably generalize this to something like:

When precompiled headers are used compiling HLSL, the ``ExternalSemaSource``
will be a ``MultiplexExternalSemaSource`` which include both the ``ASTReader``
and ``HLSLExternalSemaSource``. For Built-in declarations that are  already
completed in the serialized AST, the ``HLSLExternalSemaSource`` will reuse the
existing declarations and not introduce new declarations. If the built-in types
are not completed in the serialized AST, the ``HLSLExternalSemaSource`` will
create new declarations and connect the de-serialized decls as the previous
declaration.

[python3kgae]

Update to match comments

clang/docs/HLSL/HLSLSupport.rst

 ----
 
 HLSL's Sema implementation will also provide an ``ExternalSemaSource``. In DXC,
 an ``ExternalSemaSource`` is used to provide definitions for HLSL built-in data
 types and built-in templates. Clang is already designed to allow an attached
 ``ExternalSemaSource`` to lazily complete data types, which is a **huge**
 performance win for HLSL.
 
+If precompiled headers are used when compiling HLSL, the ``ExternalSemaSource``
+will be a ``MultiplexExternalSemaSource`` which includes both the ``ASTReader``
+and ``HLSLExternalSemaSource``. For Built-in declarations that are already
+completed in the serialized AST, the ``HLSLExternalSemaSource`` will reuse the
+existing declarations and not introduce new declarations. If the built-in types
+are not completed in the serialized AST, the ``HLSLExternalSemaSource`` will
+create new declarations and connect the de-serialized decls as the previous
+declaration.
+
 CodeGen
 -------
 
 Like OpenCL, HLSL relies on capturing a lot of information into IR metadata.
 *hand wave* *hand wave* *hand wave* As a design principle here we want our IR to
 be idiomatic Clang IR as much as possible. We will use IR attributes wherever we
 can, and use metadata as sparingly as possible. One example of a difference from
 DXC already implemented in Clang is the use of target triples to communicate