[Support] fix TempFile infinite loop and permission denied errors

Summary:
On Windows, TempFile::create() was prone to failing with permission
denied errors when a process created many tempfiles without providing
a model large enough to accommodate them. There was also a problem
with createUniqueEntity getting into an infinite loop when all names
permitted by the model are in use. This change fixes both of these
problems and adds a unit test for them.

Reviewers: pcc, rnk, zturner

Reviewed By: zturner

Subscribers: inglorion, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D50126

llvm-svn: 338745