[ADT] Add `at` method (assertive lookup) to DenseMap and StringMap

This patch makes it easier for users when they want to use validated
lookup on DenseMap/StringMap as a composable C++ expression. For
instance:

```
// instead of
if (auto val = map.lookup(key))
   return val;
assert("...");

// we can write
return map.at(key);
```

Differential Revision: https://reviews.llvm.org/D143976

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/DenseMapTest.cpp b/llvm/unittests/ADT/DenseMapTest.cpp
index 2d01316..ba4e764 100644
--- a/llvm/unittests/ADT/DenseMapTest.cpp
+++ b/llvm/unittests/ADT/DenseMapTest.cpp
@@ -125,6 +125,10 @@ TYPED_TEST(DenseMapTest, EmptyIntMapTest) {
   EXPECT_TRUE(this->Map.find(this->getKey()) == this->Map.end());
   EXPECT_EQ(typename TypeParam::mapped_type(),
             this->Map.lookup(this->getKey()));
+
+  // LookupOrTrap tests
+  EXPECT_DEATH({ this->Map.at(this->getKey()); },
+               "DenseMap::at failed due to a missing key");
 }
 
 // Constant map tests
@@ -156,6 +160,10 @@ TYPED_TEST(DenseMapTest, SingleEntryMapTest) {
   EXPECT_TRUE(this->Map.find(this->getKey()) == this->Map.begin());
   EXPECT_EQ(this->getValue(), this->Map.lookup(this->getKey()));
   EXPECT_EQ(this->getValue(), this->Map[this->getKey()]);
+
+  // LookupOrTrap tests
+  EXPECT_DEATH({ this->Map.at(this->getKey(1)); },
+               "DenseMap::at failed due to a missing key");
 }
 
 // Test clear() method