[CMake] Allow *only* static stdlib to be built

Prior to these changes, it was only possible to build static libraries
for the standard library and SDK overlays if *also* building dynamic
libraries. This is problematic if, for example, you wanted to defer the
linking of the standard library build products until later.

These changes allow you to build *only* static libraries, using
something like the following command:

```
$ utils/build-script -R --skip-build-benchmarks \
    -- --build-swift-static-stdlib=1 --build-swift-dynamic-stdlib=0
```

Author: modocache

CMakeLists.txt

@@ -44,18 +44,34 @@ option(SWIFT_BUILD_TOOLS
     "Build the Swift compiler and other tools"
     TRUE)
 
-option(SWIFT_BUILD_STDLIB
-    "Build the Swift standard library (independent of the SDK headers)"
+option(SWIFT_BUILD_DYNAMIC_STDLIB
+    "Build dynamic variants of the Swift standard library"
     TRUE)
 
-option(SWIFT_BUILD_SDK_OVERLAY
-    "Build Swift SDK overlay"
+option(SWIFT_BUILD_STATIC_STDLIB
+    "Build static variants of the Swift standard library"
+    FALSE)
+
+if(SWIFT_BUILD_DYNAMIC_STDLIB OR SWIFT_BUILD_STATIC_STDLIB)
+  set(SWIFT_BUILD_STDLIB TRUE)
+else()
+  set(SWIFT_BUILD_STDLIB FALSE)
+endif()
+
+option(SWIFT_BUILD_DYNAMIC_SDK_OVERLAY
+    "Build dynamic variants of the Swift SDK overlays"
     TRUE)
 
-option(SWIFT_BUILD_STATIC_STDLIB
-    "Build static variants of the Swift standard library and SDK overlay"
+option(SWIFT_BUILD_STATIC_SDK_OVERLAY
+    "Build static variants of the Swift SDK overlays"
     FALSE)
 
+if(SWIFT_BUILD_DYNAMIC_SDK_OVERLAY OR SWIFT_BUILD_STATIC_SDK_OVERLAY)
+  set(SWIFT_BUILD_SDK_OVERLAY TRUE)
+else()
+  set(SWIFT_BUILD_SDK_OVERLAY FALSE)
+endif()
+
 option(SWIFT_BUILD_PERF_TESTSUITE
     "Create targets for swift performance benchmarks."
     FALSE)
@@ -795,14 +811,14 @@ if(SWIFT_BUILD_TOOLS)
 endif()
 
 is_sdk_requested("${SWIFT_HOST_VARIANT_SDK}" SWIFT_HOST_SDK_REQUESTED)
-if(SWIFT_BUILD_TOOLS AND SWIFT_BUILD_STDLIB AND SWIFT_HOST_SDK_REQUESTED)
+if(SWIFT_BUILD_TOOLS AND SWIFT_BUILD_DYNAMIC_STDLIB AND SWIFT_HOST_SDK_REQUESTED)
   add_subdirectory(tools/swift-reflection-dump)
 endif()
 
 add_subdirectory(utils)
 add_subdirectory(stdlib)
 
-if(SWIFT_BUILD_STDLIB AND SWIFT_INCLUDE_TESTS)
+if(SWIFT_BUILD_DYNAMIC_STDLIB AND SWIFT_INCLUDE_TESTS)
   add_subdirectory(tools/swift-reflection-test)
 endif()