_FoundationUnicode: apply symbol renaming for non-Darwin targets

When building on non-Darwin targets, we are not guaranteed that there is
no ICU library on the system (Windows ships an ICU since Windows 1703
(RedStone 2), SDK 10.0.15063. This now is implicitly pulled in
OneCore.lib which vends the memory API contract; Linux normally ships a
copy of ICU; android ships a copy of ICU since Android 6.0 (API Level
23) but is not part of the supported APIs). The availability of ICU on
the system and in Swift causes a conflict. As there is no stable ABI for
ICU, this is a problem and can cause spurious failures. Such a failure
has been observed on Windows when statically linking Foundation.

Unfortunately, the symbol renaming support in ICU relies on the
`U_ICU_ENTRY_POINT_RENAME` macro which is defined thusly:

  ```
  #define U_DEF_ICU_ENTRY_POINT_RENAME(x, y, z) x ## y ## z
  #define U_DEF2_ICU_ENTRY_POINT_RENAME(x, y, z) U_DEF_ICU_ENTRY_POINT_RENAME(x, y, z)
  #define U_ICU_ENTRY_POINT_RENAME(name) U_DEF2_ICU_ENTRY_POINT_RENAME(name, ICU_VERSION_SUFFIX, U_LIB_SUFFIX_C_NAME)
  ```

This macro is too complex for the clang importer to import into Swift,
making this not possible to use directly. Instead, we manually perform
the expansion of the CPP macros, using the `swift_` prefix instead to
isolate and namespace our implementation.

Unfortunately, even the aliasing macro is too complex for the clang
importer to import directly. Because changing all the call sites would
not be tractable (nor maintainable), this will require an additional
change (swiftlang/swift#81840) to support aliasing macros to be imported
into Swift.

Although in theory only the dynamically linked version should be
susceptible to the interpositioning issue, if any of the system
libraries causes ICU to be included (e.g. a linker script on Unix or an
import library pulling in another version), we can also hit this issue
in the statically linked scenario. When dynamically linking, on Windows
at least, it would not be a problem if the symbol is bound properly at
link time as the symbolic resolution involves two-level namespacing,
which is not available on ELF hosts.

On Darwin, we cannot perform the renaming as the library is vended by
the system and this would constitute an ABI break.

Author: compnerd

CMakeLists.txt

@@ -45,7 +45,7 @@ add_compile_definitions(
     $<$<COMPILE_LANGUAGE:C,CXX>:U_HAVE_STRTOD_L=1>
     $<$<COMPILE_LANGUAGE:C,CXX>:U_HAVE_XLOCALE_H=1>
     $<$<COMPILE_LANGUAGE:C,CXX>:U_HAVE_STRING_VIEW=1>
-    $<$<COMPILE_LANGUAGE:C,CXX>:U_DISABLE_RENAMING=1>
+    $<$<COMPILE_LANGUAGE:C,CXX>:U_DISABLE_RENAMING=$<IF:$<PLATFORM_ID:Darwin>,1,0>>
     $<$<COMPILE_LANGUAGE:C,CXX>:U_COMBINED_IMPLEMENTATION>
     $<$<COMPILE_LANGUAGE:C,CXX>:U_COMMON_IMPLEMENTATION>
     $<$<COMPILE_LANGUAGE:C,CXX>:U_I18N_IMPLEMENTATION>

icuSources/include/_foundation_unicode/uconfig.h

@@ -101,7 +101,7 @@
 #ifndef U_DISABLE_RENAMING
 #if APPLE_ICU_CHANGES
 //rdar://60884991 #58 Replace installsrc patching with changes directly in header files
-#define U_DISABLE_RENAMING 1
+#define U_DISABLE_RENAMING defined(__APPLE__)
 #else
 #define U_DISABLE_RENAMING 0
 #endif // APPLE_ICU_CHANGES