Tablegen: Remove the error for duplicate include files.

This error was originally added a while(7 years) ago when
including multiple files was basically always an error. Tablegen
now has preprocessor support, which allows for building nice
c/c++ style include guards. With the current error being
reported, we unfortunately need to double guard when including
files:

* In user of MyFile.td

 #ifndef MYFILE_TD
 include MyFile.td
 #endif

* In MyFile.td

 #ifndef MYFILE_TD
 #define MYFILE_TD
 ...
 #endif

Differential Revision: https://reviews.llvm.org/D70410