Build libSwiftStaticMirror as a standalone library with minimal required dependencies.

This separates it from `libSwiftScan` and allows us to build this library without building much of the rest of the compiler.

Also refactor `utils/build-parser-lib` into `utils/build-tooling-libs` which builds both SwiftSyntaxParser and SwiftStaticMirror.

include/swift-c/CommonString/CommonString.h

@@ -0,0 +1,35 @@
+//===--- CommonString.h - C API for Swift Dependency Scanning ---*- C -*-===//
+ //
+ // This source file is part of the Swift.org open source project
+ //
+ // Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+ // Licensed under Apache License v2.0 with Runtime Library Exception
+ //
+ // See https://swift.org/LICENSE.txt for license information
+ // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+ //
+ //===----------------------------------------------------------------------===//
+
+ #ifndef SWIFT_C_LIB_SWIFT_COMMON_STRING_H
+ #define SWIFT_C_LIB_SWIFT_COMMON_STRING_H
+
+ #include <stdbool.h>
+ #include <stddef.h>
+ #include <stdint.h>
+
+ /**
+  * A character string used to pass around dependency scan result metadata.
+  * Lifetime of the string is strictly tied to the object whose field it
+  * represents. When the owning object is released, string memory is freed.
+  */
+ typedef struct {
+   const void *data;
+   size_t length;
+ } swiftscan_string_ref_t;
+
+ typedef struct {
+   swiftscan_string_ref_t *strings;
+   size_t count;
+ } swiftscan_string_set_t;
+
+ #endif // SWIFT_C_LIB_SWIFT_COMMON_STRING_H