Use O_BINARY when opening GCDA file on Windows

Summary:
Fixes https://bugs.llvm.org/show_bug.cgi?id=34922.

Apparently, the mode in **fdopen** gets simply ignored and Windows only cares about the mode of the original **open**.

I have verified this both with the simple case from bug 34922 and with a full Firefox build.

Reviewers: zturner

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38984

llvm-svn: 316048