[DebugInfo] Move the SymbolizableObjectFile header to include/llvm.

This change moves the SymbolizableObjectFile header to
include/llvm/DebugInfo/Symbolize. Making this header available to other
llvm libraries simplifies use cases where implicit caching, multiple
platform support and other features of the Symbolizer class are not
required. This also makes the dependent libraries easier to unit test
by having mocks which derive from SymbolizableModule.

Differential Revision: https://reviews.llvm.org/D116781