[libclang]: Honor LIBCLANG_NOTHREADS for clang_parseTranslationUnit*

Looks like this one was forgotten for clang_parseTranslationUnit*, as
LIBCLANG_NOTHREADS is checked for/in:

clang_saveTranslationUnit()
clang_reparseTranslationUnit()
clang_codeCompleteAt()
clang_indexTranslationUnit()
clang_indexSourceFile()

Patch by Nikolai Kosjar!

Differential Revision: https://reviews.llvm.org/D36821

llvm-svn: 311990