Fix suppressions file usage

Summary:
Always quote suppressions files given to *_OPTIONS.
This will make it not break when given full Windows paths (otherwise,
parsing would stop after the drive's letter + ':').

Also fix one or two cases where the suppression files' extensions were
not *.supp.

Reviewers: samsonov, kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6680

llvm-svn: 224529