[flang] Use internal linkage for string literals

On Windows, global string literals with "linkonce" linkage is not
supported without using comdat. As a simpler fix than adding comdat
support we can use internal linkage instead.

This fixes a bug where two string literals with the same value in
different fortran files would cause a linker error due to the use
of linkonce linkage.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D149859