[clangd] Add a trained DecisionForest for code completion.

[usaxena95]

Summary

Replaces the dummy CodeCompletion model with a trained DecisionForest
model.

The features.json are manually curated specifying the features
to be used. This is a one-time cost and does not change if the model
changes until we decide to add/remove features.

Stats for comparison:

---

Binary size
ClangdTests binary: -> 263.1624MB -> 263.65324MB (*0.2%* increase)

---

Build Time
ninja ClangdTests:
Only changing the completion model and rebuilding using cache: 32.24s -> 38.75s
Clean build: 05:27 min-> 5:30 min

Minor (3-6s) increase in build time.

---

Latency
For float Evaluate(const Example&): 3-4 us.
A completion query having ~1000 completion candidates will take about 3-4ms to get ranked.

clang-tools-extra/clangd/quality/model/features.json

 [
     {
+        "name": "FilterLength",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsDeprecated",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsReservedName",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsImplementationDetail",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "NumReferences",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsNameInContext",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsForbidden",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsInBaseClass",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "FileProximityDistance",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "SemaFileProximityScore",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "SymbolScopeDistance",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "SemaSaysInScope",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "IsInstanceMember",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "HadContextType",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "HadSymbolType",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "TypeMatchesPreferred",
+        "kind": "NUMBER"
+    },
+    {
+        "name": "SymbolCategory",
+        "kind": "ENUM",
+        "type": "clang::clangd::SymbolQualitySignals::SymbolCategory",
+        "header": "Quality.h"
+    },
+    {
         "name": "ContextKind",
         "kind": "ENUM",
         "type": "clang::CodeCompletionContext::Kind",
         "header": "clang/Sema/CodeCompleteConsumer.h"
+    },
+    {
+        "name": "Scope",
+        "kind": "ENUM",
+        "type": "clang::clangd::SymbolRelevanceSignals::AccessibleScope",
+        "header": "Quality.h"
     }
 ]
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