[CMake] Use PUBLIC link mode for static libraries

Using INTERFACE prevents the use of imported libraries as we've done
in 00b3d49 because these aren't linked against the target, they're
only made part of the interface. This doesn't affect the output since
static libraries aren't being linked into, but it enables the use of
imported libraries.

Differential Revision: https://reviews.llvm.org/D74106

(cherry picked from commit 50a6d3a)

Author: petrhosek

llvm/cmake/modules/AddLLVM.cmake

@@ -602,7 +602,7 @@ function(llvm_add_library name)
   endif()
 
   if(ARG_STATIC)
-    set(libtype INTERFACE)
+    set(libtype PUBLIC)
   else()
     # We can use PRIVATE since SO knows its dependent libs.
     set(libtype PRIVATE)