[PDB] Add documentation for the PDB Stream

[zturner]

Summary

This continues the effort to the document the PDB file format. This documents the first important stream, the PDB Stream.

[amccarth]

Is the header the only thing in the stream? The word "header" suggests that there's something more after it, but this seems to describe only the header.

docs/PDB/PdbStream.rst

54

So a brand new PDB has an Age of 1 not 0?

69

Did you close the parentheses too soon? I assume you only increment the Age when updating a PDB. Otherwise, you'd just be initializing the Age, right?

docs/PDB/PdbStream.rst

 ========================================
 The PDB Info Stream (aka the PDB Stream)
 ========================================
+
+.. contents::
+   :local:
+
+.. _info_header:
+
+Stream Header
+=============
+At offset 0 of the PDB Stream is a header with the following layout:
+
+
+.. code-block:: c++
+
+  struct PdbStreamHeader {
+    ulittle32_t Version;
+    ulittle32_t Signature;
+    ulittle32_t Age;
+    Guid UniqueId;
+  };
+
+- **Version** - A Value from the following enum:
+
+.. code-block:: c++
+
+  enum class PdbStreamVersion : uint32_t {
+    VC2 = 19941610,
+    VC4 = 19950623,
+    VC41 = 19950814,
+    VC50 = 19960307,
+    VC98 = 19970604,
+    VC70Dep = 19990604,
+    VC70 = 20000404,
+    VC80 = 20030901,
+    VC110 = 20091201,
+    VC140 = 20140508,
+  };
+
+While the meaning of this field appears to be obvious, in practice we have
+never observed a value other than ``VC70``, even with modern versions of
+the toolchain, and it is unclear why the other values exist.  It is assumed
+that certain aspects of the PDB stream's layout, and perhaps even that of
+the other streams, will change if the value is something other than ``VC70``.
+
+- **Signature** - A 32-bit time-stamp generated with a call to ``time()`` at
+  the time the PDB file is written.  Note that due to the inherent uniqueness
+  problems of using a timestamp with 1-second granularity, this field does not
+  really serve its intended purpose, and as such is typically ignored in favor
+  of the ``Guid`` field, described below.
+  
+- **Age** - The number of times the PDB file has been written.  This can be used
+  along with ``Guid`` to match the PDB to its corresponding executable.
+  
+- **Guid** - A 128-bit identifier guaranteed to be unique across space and time.
+  In general, this can be thought of as the result of calling the Win32 API 
+  `UuidCreate <https://msdn.microsoft.com/en-us/library/windows/desktop/aa379205(v=vs.85).aspx>`__,
+  although LLVM cannot rely on that, as it must work on non-Windows platforms.
+  
+Matching a PDB to its executable
+================================
+The linker is responsible for writing both the PDB and the final executable, and
+as a result is the only entity capable of writing the information necessary to
+match the PDB to the executable.
+
+In order to accomplish this, the linker generates a guid for the PDB (or
+re-uses the existing guid if it is linking incrementally) and increments the Age
+field.
+
+The executable is a PE/COFF file, and part of a PE/COFF file is the presence of
+number of "directories".  For our purposes here, we are interested in the "debug
+directory".  The exact format of a debug directory is described by the
+`IMAGE_DEBUG_DIRECTORY structure <https://msdn.microsoft.com/en-us/library/windows/desktop/ms680307(v=vs.85).aspx>`__.
+For this particular case, the linker emits a debug directory of type
+``IMAGE_DEBUG_TYPE_CODEVIEW``.  The format of this record is defined in
+``llvm/DebugInfo/CodeView/CVDebugRecord.h``, but it suffices to say here only
+that it includes the same ``Guid`` and ``Age`` fields.  At runtime, a
+debugger or tool can scan the COFF executable image for the presence of
+a debug directory of the correct type and verify that the Guid and Age match.