COFF: Assign the correct symbol type to internal functions.

The COFFSymbolRef::isFunctionDefinition() function tests for several conditions
that are not related to whether a symbol is a function, but rather whether
the symbol meets the requirements for a function definition auxiliary record,
which excludes certain symbols such as internal functions and undefined
references. The test we need to determine the symbol type is much simpler:
we only need to compare the complex type against IMAGE_SYM_DTYPE_FUNCTION.

llvm-svn: 244195