Don't search for IMPORTED_LOCATION of INTERFACE_LIBRARY (14636)

The INTERFACE_LIBRARY type does not have any LOCATION at all, so
return early from GetMappedConfig. GetMappedConfig is called from
two locations, one of which already pre-checks the INTERFACE_LIBRARY
case. Remove that pre-check and handle that case inside the method
instead.

Source/cmTarget.cxx

@@ -4860,6 +4860,15 @@ bool cmTarget::GetMappedConfig(std::string const& desired_config,
                                const char** imp,
                                std::string& suffix) const
 {
+  if (this->GetType() == INTERFACE_LIBRARY)
+    {
+    // This method attempts to find a config-specific LOCATION for the
+    // IMPORTED library. In the case of INTERFACE_LIBRARY, there is no
+    // LOCATION at all, so leaving *loc and *imp unchanged is the appropriate
+    // and valid response.
+    return true;
+    }
+
   // Track the configuration-specific property suffix.
   suffix = "_";
   suffix += desired_config;
@@ -4992,8 +5001,7 @@ void cmTarget::ComputeImportInfo(std::string const& desired_config,
   const char* loc = 0;
   const char* imp = 0;
   std::string suffix;
-  if (this->GetType() != INTERFACE_LIBRARY &&
-      !this->GetMappedConfig(desired_config, &loc, &imp, suffix))
+  if (!this->GetMappedConfig(desired_config, &loc, &imp, suffix))
     {
     return;
     }

Tests/RunCMake/interface_library/RunCMakeTest.cmake

@@ -4,3 +4,4 @@ run_cmake(invalid_name)
 run_cmake(target_commands)
 run_cmake(no_shared_libs)
 run_cmake(whitelist)
+run_cmake(genex_link)

Tests/RunCMake/interface_library/genex_link-result.txt

@@ -0,0 +1 @@
+0

Tests/RunCMake/interface_library/genex_link-stderr.txt

@@ -0,0 +1 @@
+^$

Tests/RunCMake/interface_library/genex_link.cmake

@@ -0,0 +1,22 @@
+
+cmake_minimum_required(VERSION 2.8.12.20131125 FATAL_ERROR)
+
+project(genex_link)
+
+set(_main_cpp ${CMAKE_CURRENT_BINARY_DIR}/main.cpp)
+file(WRITE ${_main_cpp}
+  "int main(int argc, char** argv) { return 0; }\n"
+)
+
+add_library(foo::bar INTERFACE IMPORTED)
+set_target_properties(foo::bar
+  PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "${CMAKE_CURRENT_SOURCE_DIR}"
+    # When not using a generator expression here, no error is generated
+    INTERFACE_LINK_LIBRARIES "$<$<NOT:$<CONFIG:DEBUG>>:foo_bar.lib>"
+)
+
+add_executable(main ${_main_cpp})
+target_include_directories(main PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}")
+
+target_link_libraries(main foo::bar)