Introduce an 'external_source_symbol' attribute that describes the origin

and the nature of a declaration

This commit adds an external_source_symbol attribute to Clang. This attribute
specifies that a declaration originates from an external source and describes
the nature of that source. This attribute will be used to improve IDE features
like 'jump-to-definition' for mixed-language projects or project that use
auto-generated code.

rdar://30423368

Differential Revision: https://reviews.llvm.org/D29819

llvm-svn: 296649