[libclang] In ASTUnit::Parse copy the CompilerInvocation object instead of

modifying directly for the preamble.

This avoids an awful, hard to find, bug where "PreprocessorOpts.DisablePCHValidation = true"
would be persistent for subsequent reparses of the translation unit which would result
in defines, present in command-line but not in the PCH, being ignored.

Fixes rdar://9615399.

llvm-svn: 139512