[Driver] Avoid invalidated iterator in insertTargetAndModeArgs

Doing an .insert() can potentially invalidate iterators by reallocating the
vector's storage. When all the stars align just right, this causes segfaults
or glibc aborts.

Gentoo Linux bug (crashes while building Chromium): https://bugs.gentoo.org/650082.

Patch by Hector Martin!

Differential Revision: https://reviews.llvm.org/D44607

llvm-svn: 327863