[libclang] Allow building a precompiled preamble with compiler errors

A while ago we allowed libclang to build a PCH that had compiler errors; this was to retain the performance
afforded by a PCH even if the user's code is in an intermediate state.

Extend this for the precompiled preamble as well.

rdar://14109828

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183717 91177308-0d34-0410-b5e6-96231b3b80d8