We are using patched libc++ which uses raw pointers for vector itrators
to improve code compilation speed. This commit fixed two compilation
issues in toml11:
* location::const_iterator deinition assumes that vector const_iterator
is struct or class type rather than raw pointer.
* `const const_itetr foo()` triggers `-Wignored-qualifiers` for primitive
types and `void` which breaks `-Wextra -Werror` compilation.
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