target_link_libraries: Fix regression in case of $<CONFIG> genex

Since commit b8626261e9 (Precompile headers: Add methods to generate PCH
sources, 2019-07-13, v3.16.0-rc1~182^2~4) we look up source files for a
target using an upper-case configuration even though an original-case
name is sufficient.  Since commit 36ded610af (PCH: Generate sources
during Compute step, 2019-10-05, v3.16.0-rc1~2^2) the source file lookup
is the first time we compute many on-demand structures that depend on
the configuration name.  This caused the `$<CONFIG>` generator
expression to evaluate to the upper-case configuration name in some
cases where we used original-case before.

Fix this by switching the source file lookup to the original-case config
name.  Add a test covering the symptom that led to the discovery of this
problem.

Fixes: #20517

Source/cmLocalGenerator.cxx

@@ -2429,11 +2429,9 @@ void cmLocalGenerator::AddPchDependencies(cmGeneratorTarget* target)
   }
 
   for (std::string const& config : configsList) {
-    const std::string buildType = cmSystemTools::UpperCase(config);
-
     // FIXME: Refactor collection of sources to not evaluate object libraries.
     std::vector<cmSourceFile*> sources;
-    target->GetSourceFiles(sources, buildType);
+    target->GetSourceFiles(sources, config);
 
     for (const std::string& lang : { "C", "CXX", "OBJC", "OBJCXX" }) {
       auto langSources = std::count_if(
@@ -2602,15 +2600,13 @@ void cmLocalGenerator::AddUnityBuild(cmGeneratorTarget* target)
     config = this->Makefile->GetSafeDefinition("CMAKE_BUILD_TYPE");
   }
 
-  const std::string buildType = cmSystemTools::UpperCase(config);
-
   std::string filename_base =
     cmStrCat(this->GetCurrentBinaryDirectory(), "/CMakeFiles/",
              target->GetName(), ".dir/Unity/");
 
   // FIXME: Refactor collection of sources to not evaluate object libraries.
   std::vector<cmSourceFile*> sources;
-  target->GetSourceFiles(sources, buildType);
+  target->GetSourceFiles(sources, config);
 
   auto batchSizeString = target->GetProperty("UNITY_BUILD_BATCH_SIZE");
   const size_t unityBatchSize =

Tests/RunCMake/target_link_libraries/ConfigCase-result.txt

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

Tests/RunCMake/target_link_libraries/ConfigCase-stderr.txt

@@ -0,0 +1,13 @@
+^CMake Error at ConfigCase.cmake:[0-9]+ \(add_library\):
+  Target "impl" links to target "config::impl-Debug" but the target was not
+  found.  Perhaps a find_package\(\) call is missing for an IMPORTED target, or
+  an ALIAS target is missing\?
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)
++
+CMake Error at ConfigCase.cmake:[0-9]+ \(add_library\):
+  Target "impl" links to target "config::iface-Debug" but the target was not
+  found.  Perhaps a find_package\(\) call is missing for an IMPORTED target, or
+  an ALIAS target is missing\?
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)

Tests/RunCMake/target_link_libraries/ConfigCase.cmake

@@ -0,0 +1,6 @@
+cmake_policy(VERSION 3.15)
+enable_language(C)
+add_library(iface INTERFACE)
+target_link_libraries(iface INTERFACE "config::iface-$<CONFIG>")
+add_library(impl empty.c)
+target_link_libraries(impl PRIVATE "config::impl-$<CONFIG>" iface)

Tests/RunCMake/target_link_libraries/RunCMakeTest.cmake

@@ -1,5 +1,13 @@
 include(RunCMake)
 
+if(RunCMake_GENERATOR_IS_MULTI_CONFIG)
+  set(RunCMake_TEST_OPTIONS -DCMAKE_CONFIGURATION_TYPES=Debug)
+else()
+  set(RunCMake_TEST_OPTIONS -DCMAKE_BUILD_TYPE=Debug)
+endif()
+run_cmake(ConfigCase)
+unset(RunCMake_TEST_OPTIONS)
+
 run_cmake(CMP0023-WARN)
 run_cmake(CMP0023-NEW)
 run_cmake(CMP0023-WARN-2)