[MS] Add frontend support for __declspec(allocator)

The intention is to add metadata to direct call sites of functions
marked with __declspec(allocator), which will ultimately result in some
S_HEAPALLOCSITE debug info records when emitting codeview.

This is a piece of PR38491

llvm-svn: 356964
diff --git a/clang/test/SemaCXX/declspec-allocator.cpp b/clang/test/SemaCXX/declspec-allocator.cpp
new file mode 100644
index 0000000..e1af497
--- /dev/null
+++ b/clang/test/SemaCXX/declspec-allocator.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fms-compatibility -triple x86_64-windows-msvc -std=c++14 -fms-extensions -fms-compatibility-version=19.00 -verify %s
+
+__declspec(allocator) int err_on_data; // expected-warning {{'allocator' attribute only applies to functions}}
+__declspec(allocator) struct ErrOnStruct1; // expected-warning {{place it after "struct" to apply attribute}}
+struct __declspec(allocator) ErrOnStruct2 {}; // expected-warning {{'allocator' attribute only applies to functions}}
+__declspec(allocator) void err_on_ret_void(); // expected-warning {{not a pointer or reference type}}
+__declspec(allocator) int err_on_ret_int(); // expected-warning {{not a pointer or reference type}}
+__declspec(allocator) void *accept_on_ptr1();
+__declspec(allocator) void *accept_on_ptr2(size_t);
+void * __declspec(allocator) accept_on_ptr3(size_t); // expected-error {{expected unqualified-id}}
+
+struct Foo { int x; };
+__declspec(allocator) Foo *accept_nonvoid_ptr(size_t);