CppTools: Auto-include pre-compiled headers

So far the pre-compiled headers were processed (thus defines from those
headers were visible), but the actual includes for the documents were
not added, which is necessary for lookup/completion.

Note that this will be only done if pre-compiled headers are not ignored
(Options > C++ > Code Model > [] Ignore pre-compiled headers).

Change-Id: I54a8e6b00597af164d958e3e9f2a1075ea187788
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

tests/cppmodelmanager/testdata_defines/pch1.h

@@ -1 +1,2 @@
 #define SUB1
+class ClassInPch1{};

tests/cppmodelmanager/testdata_defines/pch2.h

@@ -1 +1,2 @@
 #define SUB2
+class ClassInPch2{};