Add clang_free to libclang to free memory allocated in libclang.

One of the problems libclang tests has running under Windows is memory
allocated in libclang.dll but being freed in the test executable, possibly
by a different memory manager. This patch exposes a new export function,
clang_free(), used to free any allocated memory with the same libclang.dll
memory manager that allocated the memory.

http://reviews.llvm.org/D10949

Reviewed by Reid Kleckner, Douglas Gregor.

llvm-svn: 241789