Polly library targets already disable PCH reuse because Polly
unconditionally builds with -fno-rtti and -fno-exceptions. Reusing LLVM
PCHs that were built with RTTI or exceptions enabled is incompatible
with Clang when compiling Polly targets under those flags.
After 47eb8b43c9 enabled PCH reuse for unit tests, Polly unit tests
can hit the same mismatch as the library targets. Pass DISABLE_PCH_REUSE
through the shared add_polly_unittest wrapper so all Polly unit tests
follow the existing Polly target policy.
cc @aengelke -- a minor fix for polly.
d629a22170