CppTools: Allow interpreting ambiguous headers as C headers

...instead of C++ headers.

For the Clang Code Model this results in using "-x c-header" instead of
"-x c++-header".

This introduces a new option in Options > C++ > "Code Model" to
configure this.

Change-Id: I8a0ce8fa6155f5ef58743ebc7f1d0b500fbf6599
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/cpptools/cpptools.qbs

@@ -61,6 +61,7 @@ Project {
             "cppfunctionsfilter.cpp", "cppfunctionsfilter.h",
             "cppincludesfilter.cpp", "cppincludesfilter.h",
             "cppindexingsupport.cpp", "cppindexingsupport.h",
+            "cpplanguage.h",
             "cpplocalsymbols.cpp", "cpplocalsymbols.h",
             "cpplocatordata.cpp", "cpplocatordata.h",
             "cpplocatorfilter.cpp", "cpplocatorfilter.h",