[llvm-nm] Print 'I' for import table data in COFF

The character gets uppercased into 'I' when it's a global symbol.

In GNU binutils, nm prints 'I' for symbols classified by
bfd_is_ind_section - which probably isn't exactly/only import
tables.

When building for win32, (some incarnations of?) libtool has got
rules that try to inspect linked libraries, and in order to
be sure that it is linking to a DLL import library as opposed to
a static library, it expects to find the string " I " in the output
of $NM when run on such an import library.

GNU binutils nm also flags all of the .idata$X chunks as 'i' (while
this patch only makes it set on .idata$2 and .idata$6) and also
flags __imp__function as 'I'.

Differential Revision: https://reviews.llvm.org/D39540

llvm-svn: 317300