PCH: Fix logic error that incorrectly clears sources during VS generation

Since commit 729d997f10 (Precompile Headers: Add REUSE_FROM signature,
2019-08-30, v3.16.0-rc1~101^2), `GetPchFileObject` handles the case that
it is called first for another target's `REUSE_FROM` by calling
`AddSource` to make sure `GetObjectName` can produce the correct object
name.  However, `AddSource` causes `ClearSourcesCache` to be called,
which since commit a9f4f58f0c (cmGeneratorTarget: Clear AllConfigSources
in ClearSourcesCache, 2020-05-15, v3.16.7~2^2) now correctly erases the
`AllConfigSources` structure.  This is okay during `AddPchDependencies`,
but there is another code path in which it is problematic.

When the Visual Studio generator's `WriteAllSources` method is looping
over the sources, the `cmake_pch.cxx` source is encountered first.  This
causes `OutputSourceSpecificFlags` to call `GetPchCreateCompileOptions`,
which calls `GetPchFile`, which under MSVC with `CMAKE_LINK_PCH` calls
`GetPchFileObject`.  That leads to `ClearSourcesCache` erasing the
structure over which `WriteAllSources` is iterating!

This bug is caught by our `RunCMake.PrecompileHeaders` test when run
with the VS generator as of the commit that exposed it by fixing
`ClearSourcesCache`.  However, that change was backported to the CMake
3.16 series after testing only with later versions versions that contain
commit a55df20499 (Multi-Ninja: Add precompile headers support,
2020-01-10, v3.17.0-rc1~136^2).  By adding proper multi-config support
for PCH, that commit taught `cmLocalGenerator::AddPchDependencies` to
call `GetPchFile` with the real set of configurations instead of just
the empty string.  This allows the `GetPchFile` cache of PCH sources to
be populated up front so that the later calls to it in the
`WriteAllSources` loop as described above do not actually call
`GetPchFileObject` or `ClearSourcesCache`.  That hid the problem.

Fix this by re-ordering calls to `AddPchDependencies` to handle
`REUSE_FROM` targets only after the targets whose PCH they re-use.
Remove the now-unnecessary call to `AddSource` from `GetPchFileObject`
so that `ClearSourcesCache` is never called during `WriteAllSources`.
Update the PchReuseFrom test case to cover an ordering of targets that
causes generators to encounter a `REUSE_FROM` target before the target
whose PCH it re-uses.

Fixes: #20770

Tests/RunCMake/PrecompileHeaders/PchReuseFrom.cmake

@@ -5,6 +5,11 @@ if(CMAKE_C_COMPILE_OPTIONS_USE_PCH)
   add_definitions(-DHAVE_PCH_SUPPORT)
 endif()
 
+# Add this before the target from which we will reuse the PCH
+# to test that generators can handle reversed ordering.
+add_library(foo foo.c)
+target_include_directories(foo PUBLIC include)
+
 add_library(empty empty.c)
 target_precompile_headers(empty PRIVATE
   <stdio.h>
@@ -12,8 +17,6 @@ target_precompile_headers(empty PRIVATE
 )
 target_include_directories(empty PUBLIC include)
 
-add_library(foo foo.c)
-target_include_directories(foo PUBLIC include)
 target_precompile_headers(foo REUSE_FROM empty)
 
 # should not cause problems if configured multiple times