Provide reason messages for unviable inlining

InlineCost's isInlineViable() is changed to return InlineResult
instead of bool. This provides messages for failure reasons and
allows to get more specific messages for cases where callsites
are not viable for inlining.

Reviewed By: xbolva00, anemet

Differential Revision: https://reviews.llvm.org/D57089

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352849 91177308-0d34-0410-b5e6-96231b3b80d8

test/Frontend/optimization-remark-with-hotness.c

@@ -66,7 +66,7 @@ void bar(int x) {
 
 int main(int argc, const char *argv[]) {
   for (int i = 0; i < 30; i++)
-    // expected-remark@+1 {{bar not inlined into main because it should never be inlined (cost=never): always inliner (hotness:}}
+    // expected-remark@+1 {{bar not inlined into main because it should never be inlined (cost=never): no alwaysinline attribute (hotness:}}
     bar(argc);
   return sum;
 }