Add RestrictQualifierLoc to DeclaratorChunk::FunctionTypeInfo

Clang supports __restrict__ as a function qualifier, but
DeclaratorChunk::FunctionTypeInfo lacked a field to track the qualifier's
source location (as we do with volatile, etc.). This was the subject of a FIXME
in GetFullTypeForDeclarator (in SemaType.cpp). This should also prove useful as
we add more warnings regarding questionable uses of the restrict qualifier.

There is no significant functional change (except for an improved source range
associated with the err_invalid_qualified_function_type diagnostic fixit
generated by GetFullTypeForDeclarator).