Attempt to fix null ASTContext in ASTUnit error path

We don't need the ASTContext for the diagnostics, only the language
options, which we can get from the compiler invocation. It worries me
how many categorically different states the ASTUnit class can be in
depending on how it is being constructed/used.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206909 91177308-0d34-0410-b5e6-96231b3b80d8

• include/clang/Frontend/ASTUnit.h[diff]
• lib/Frontend/ASTUnit.cpp[diff]
• tools/libclang/CIndex.cpp[diff]