libclang: add clang_CXXMethod_isConst API that allows to determine if a C++
member function or member function template is declared 'const'

Patch by Kevin Funk with testcase updates by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205714 91177308-0d34-0410-b5e6-96231b3b80d8