Add internal_lld Lit-test feature. (#2952)

This prevents test failures when LDC is built without internal LLD.

tests/linking/link_internally.d

@@ -1,4 +1,5 @@
 // REQUIRES: Windows
+// REQUIRES: internal_lld
 
 // RUN: %ldc -link-internally -run %s
 

tests/lit.site.cfg.in

@@ -25,6 +25,7 @@ config.dynamic_compile     = @LDC_DYNAMIC_COMPILE@
 config.plugins_supported   = "@LDC_ENABLE_PLUGINS@" == "ON"
 config.gnu_make_bin        = "@GNU_MAKE_BIN@"
 config.ldc_host_arch       = "@LDC_HOST_ARCH@"
+config.ldc_with_lld        = "@LDC_WITH_LLD@"
 
 config.name = 'LDC'
 
@@ -124,6 +125,9 @@ elif (platform.system() == 'Windows'):
 if canDoLTO:
     config.available_features.add('LTO')
 
+if config.ldc_with_lld:
+    config.available_features.add('internal_lld')
+
 config.target_triple = '(unused)'
 
 # test_exec_root: The root path where tests should be run.