versioner: properly handle extern "C", "C++".

extern "C" and "C++" are parsed as a LinkageSpecDecl with the real Decl
as a child node. This leads to the preprocessor sticking its guard
between the extern specifier and the declaration.

Update the AST visitor to add a special-case for calculating the
SourceRange on a LinkageSpecDecl, and add a test.

Bug: https://github.com/android-ndk/ndk/issues/440
Test: python run_tests.py
Change-Id: I76445fe366cef46cfd2f16fb93d534d410c5edca