Eliminate clang_setUseExternalASTGeneration() from libclang. Between

improvements to the compiler and the introduction of crash recovery,
it no longer makes sense to allow this mode. Moreover, this eliminates
one use of the "clang" executable from within libclang; we'd like them
all to go away.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116207 91177308-0d34-0410-b5e6-96231b3b80d8