WinAdapter: Export symbols for CoTaskMem{Alloc,Free}

Some functions return lists allocated with CoTaskMemAlloc.  Applications
using libdxcompiler.so currently have to look into the code to
understand that these should be deallocated with free() instead of
delete[].  Export the CoTaskMemFree function so that there is no
guesswork involved anymore: the application can now call this function
and always free it in the right way.

include/dxc/Support/WinAdapter.h

@@ -42,9 +42,6 @@
 #define C_ASSERT(expr) static_assert((expr), "")
 #define ATLASSERT assert
 
-#define CoTaskMemAlloc malloc
-#define CoTaskMemFree free
-
 #define ARRAYSIZE(array) (sizeof(array) / sizeof(array[0]))
 
 #define _countof(a) (sizeof(a) / sizeof(*(a)))
@@ -946,6 +943,11 @@ extern "C" DXC_API_IMPORT BSTR __stdcall SysAllocStringLen(const OLECHAR *strIn,
 extern "C" DXC_API_IMPORT UINT __stdcall SysStringByteLen(BSTR bstr);
 extern "C" DXC_API_IMPORT UINT __stdcall SysStringLen(BSTR pbstr);
 
+//===-------------------------- CoTask Allocation -------------------------===//
+
+extern "C" DXC_API_IMPORT LPVOID __stdcall CoTaskMemAlloc(SIZE_T cb);
+extern "C" DXC_API_IMPORT void __stdcall CoTaskMemFree(LPVOID pv);
+
 //===--------------------- UTF-8 Related Types ----------------------------===//
 
 // Code Page

lib/DxcSupport/WinAdapter.cpp

@@ -86,6 +86,12 @@ DXC_API_IMPORT UINT __stdcall SysStringLen(BSTR pbstr) {
   return SysStringByteLen(pbstr) / 4;
 }
 
+//===-------------------------- CoTask Allocation -------------------------===//
+
+DXC_API_IMPORT LPVOID __stdcall CoTaskMemAlloc(SIZE_T cb) { return malloc(cb); }
+
+DXC_API_IMPORT void __stdcall CoTaskMemFree(LPVOID pv) { free(pv); }
+
 //===---------------------- Char converstion ------------------------------===//
 
 const char *CPToLocale(uint32_t CodePage) {