[cmake] Recover original CMAKE_C/CXX_COMPILER after changing them.

Seems that the change in the two variables was spilling into the other
target of the file, but returning it back to the original values seems
to avoid that issue.

This should unbreak the Android CI build. In it, the Linux static
library was changing to the host compiler, and that compiler was being
used for the Android runtime library, which would have never compile
that way (since the host compiler in CI is an old-ish Clang without the
necessary argument).

Author: drodriguez

stdlib/public/Reflection/CMakeLists.txt

@@ -34,6 +34,8 @@ if(SWIFT_INCLUDE_TOOLS)
   endif()
 
   if(NOT SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+    set(CURRENT_CMAKE_C_COMPILER ${CMAKE_C_COMPILER})
+    set(CURRENT_CMAKE_CXX_COMPILER ${CMAKE_CXX_COMPILER})
     set(CMAKE_C_COMPILER ${HOST_CMAKE_C_COMPILER})
     set(CMAKE_CXX_COMPILER ${HOST_CMAKE_CXX_COMPILER})
   endif()
@@ -44,5 +46,9 @@ if(SWIFT_INCLUDE_TOOLS)
     ${SWIFT_RUNTIME_CXX_FLAGS})
   set_property(TARGET swiftReflection
     APPEND_STRING PROPERTY LINK_FLAGS ${SWIFT_RUNTIME_LINK_FLAGS})
-endif()
 
+  if(NOT SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+    set(CMAKE_C_COMPILER ${CURRENT_CMAKE_C_COMPILER})
+    set(CMAKE_CXX_COMPILER ${CURRENT_CMAKE_CXX_COMPILER})
+  endif()
+endif()

stdlib/public/SwiftRemoteMirror/CMakeLists.txt

@@ -22,6 +22,8 @@ if(SWIFT_INCLUDE_TOOLS)
   endif()
 
   if(NOT SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+    set(CURRENT_CMAKE_C_COMPILER ${CMAKE_C_COMPILER})
+    set(CURRENT_CMAKE_CXX_COMPILER ${CMAKE_CXX_COMPILER})
     set(CMAKE_C_COMPILER ${HOST_CMAKE_C_COMPILER})
     set(CMAKE_CXX_COMPILER ${HOST_CMAKE_CXX_COMPILER})
   endif()
@@ -32,5 +34,9 @@ if(SWIFT_INCLUDE_TOOLS)
     ${SWIFT_RUNTIME_CXX_FLAGS})
   set_property(TARGET swiftRemoteMirror APPEND_STRING PROPERTY LINK_FLAGS
     ${SWIFT_RUNTIME_LINK_FLAGS})
-endif()
 
+  if(NOT SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+    set(CMAKE_C_COMPILER ${CURRENT_CMAKE_C_COMPILER})
+    set(CMAKE_CXX_COMPILER ${CURRENT_CMAKE_CXX_COMPILER})
+  endif()
+endif()