Merge #19413: refactor: Remove confusing BlockIndex global

<pre>Merge #19413: refactor: Remove confusing BlockIndex global fa0dfdf447d5b84a1849dc823d8508463600136a refactor: Remove confusing BlockIndex global (MarcoFalke) Pull request description: The global `::BlockIndex()` is problematic for several reasons: * It returns a mutable reference to the block tree, without the appropriate lock annotation (`m_block_index` is guarded by `cs_main`). The current code is fine, but in the future this might lead to accidental races and data corruption. * The rpc server shouldn't rely on node globals, but rather a context that is passed in to the RPC method. * Tests might want to spin up their own block tree, and thus should also not rely on a single global. Fix all issues by removing the global ACKs for top commit: promag: Code review ACK fa0dfdf447d5b84a1849dc823d8508463600136a. jonatack: re-ACK fa0dfdf Tree-SHA512: 8f158fc5e1c67e73588a21c25677b3fa0fe442313b13ec24b87054806c59607d6ba0c062a865ce3e0ee568706bd0d1faa84febda21aff5bcd65dab172f74c52f</pre>

Merge #19333: refactor: Fix clang compile failure

<pre>Merge #19333: refactor: Fix clang compile failure fa3b35a189c4a4fd9667ef0af1c7059471ac8b01 ci: Add test for clang-3.8 C++11 support (MarcoFalke) faa7431fee45b26f7ac2f5fd0b8874cb6afafbd4 refactor: Fix clang compile failure (MarcoFalke) Pull request description: Fix ``` script/standard.cpp:278:22: error: default initialization of an object of const type 'const (anonymous namespace)::CScriptVisitor' without a user-provided default constructor const CScriptVisitor g_script_visitor; ^ {} 1 error generated. ACKs for top commit: laanwj: ACK fa3b35a189c4a4fd9667ef0af1c7059471ac8b01 Tree-SHA512: b3251208945b44530224aadbc10fef1260b479c0b43a5e345501fbfd3579a9fe354b946090e023232852bbb99759da4429b58b137b7b286ddac6bd7960851f7f</pre>