[build system] Renamed the preset for LLDB_Embedded and added more comments.

This should make it less confusing to figure out what this preset is for, and
what configuration it requires.

Author: scallanan

utils/build-presets.ini

@@ -573,24 +573,28 @@ darwin-toolchain-version=%(darwin_toolchain_version)s
 #===------------------------------------------------------------------------===#
 # LLDB build configurations
 #
-# Use to build the compiler embedded in LLDB.
-# This is only used from Xcode, and expects a swift_install_destdir argument.
+# Use to build the compiler sources nested in LLDB.
+#
+# This is only used from Xcode, and expects the following configuration:
+## an argument, swift_install_destdir=[path for Swift installed products]
+## an environment variable, SWIFT_SOURCE_ROOT=[parent of swift/]
+## an environment variable, SWIFT_BUILD_ROOT=[parent of Swift build directory]
 #===------------------------------------------------------------------------===#
 
-[preset: LLDB_Embedded]
+[preset: LLDB_Nested]
 dash-dash
 build-swift-perf-testsuite=0
 install-destdir=%(swift_install_destdir)s
 
 [preset: LLDB_Swift_DebugAssert]
 mixin-preset=
-    LLDB_Embedded
+    LLDB_Nested
 
 assertions
 
 [preset: LLDB_Swift_ReleaseAssert]
 mixin-preset=
-    LLDB_Embedded
+    LLDB_Nested
 
 release-debuginfo
 assertions