[libclang] Allow skipping function bodies in preamble only

As an addition to CXTranslationUnit_SkipFunctionBodies, provide the
new option CXTranslationUnit_LimitSkipFunctionBodiesToPreamble,
which constraints the skipping of functions bodies to the preamble
only. Function bodies in the main file are not affected if this
option is set.

Skipping function bodies only in the preamble is what clangd already
does and the introduced flag implements it for libclang clients.

Patch by Nikolai Kosjar.

Differential Revision: https://reviews.llvm.org/D45815

llvm-svn: 332578