[PCH] Reading expressions from attributes should be done using ReadExpr(),
not ReadExpr().

Also add a test case making sure the thread safety attributes work as expected
when they come from a PCH.

Fixes rdar://12584141 & http://llvm.org/PR13982

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168017 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed