[cxx-interop] Re-work control flow when checking for duplicate functions.

Also add back `string.h` as a valid header and ban imports from `_SwiftConcurrencyShims`.

Author: zoecarver

lib/ClangImporter/ImportDecl.cpp

@@ -3211,16 +3211,20 @@ namespace {
               d->getName() == "cos" || d->getName() == "exit";
         };
         if (decl->getOwningModule() &&
-            decl->getOwningModule()
-                     ->getTopLevelModule()
-                     ->getFullModuleName() == "std" &&
-            isAlternativeCStdlibFunctionFromTextualHeader(decl)) {
+            (decl->getOwningModule()
+                ->getTopLevelModule()
+                ->getFullModuleName() == "std" ||
+             decl->getOwningModule()
+                ->getTopLevelModule()
+                ->getFullModuleName() == "_SwiftConcurrencyShims")) {
+          if (isAlternativeCStdlibFunctionFromTextualHeader(decl)) {
+            return nullptr;
+          }
           auto filename =
               Impl.getClangPreprocessor().getSourceManager().getFilename(
                   decl->getLocation());
           if (filename.endswith("cmath") || filename.endswith("math.h") ||
-              filename.endswith("stdlib.h") || filename.endswith("cstdlib") ||
-              filename.endswith("string.h")) {
+              filename.endswith("stdlib.h") || filename.endswith("cstdlib")) {
             return nullptr;
           }
         }

test/Interop/Cxx/stdlib/avoid-import-cxx-math.swift

@@ -6,21 +6,13 @@ import CxxStdlib
 
 func test() {
     let x: Float = 1.0
-    let y: Double = 2.0
+    let y: Float = 2.0
 
-    // Note: we dispatch `pow(Float,Double)`
-    // to ensure we don't pick up the
-    // C++ stdlib `pow` function template.
-    // The `pow` function is still reexported
-    // from Darwin via CxxStdlib, so there are
-    // matching overloads that can be found still.
-    // Note: the error is different on Glibc instead
-    // of Darwin, so do not check the exact error.
-    let _ =  CxxStdlib.pow(x, y) // expected-error {{}}
+    let _ =  pow(x, y)
 
-    let _ = CxxStdlib.abs(x) // expected-error {{module 'CxxStdlib' has no member named 'abs'}}
-    let _ = CxxStdlib.div(x) // expected-error {{module 'CxxStdlib' has no member named 'div'}}
-    let _ = CxxStdlib.strstr("a", "aaa") // expected-error {{module 'CxxStdlib' has no member named 'strstr'}}
+    let _ = abs(x)
+    let _ = div(42, 2)
+    let _ = strstr("a", "aaa")
 
     exit(0)
 }