ASTMatchers.h: Fix ODR violations by avoiding internal linkage variables in headers

Internal linkage variables ODR referenced from inline functions create
ODR violations (the same inline function ends up having different
definitions in each TU, since it references different variables - rather
than one definition).

This also happens to break modular code generation - so this is the last
fix to allow clang to compile with modular code generation.

llvm-svn: 318304