[ELF] Change isSectionData to exclude SHF_EXECINSTR

Summary:
This affects what sections are displayed as "DATA" in llvm-objdump.
The other user llvm-size is unaffected.

Before, a "TEXT" section is also "DATA", which seems weird.
The sh_flags condition matches that of bfd's SEC_DATA but the sh_type
condition uses (== SHF_PROGBITS) instead of bfd's (!= SHT_NOBITS).
bfd's SEC_DATA is not appealing as so many sections will be shown as DATA.

Reviewers: jyknight, Bigcheese

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48472

llvm-svn: 335405