Move TypeCompleter and closely related helper functions to their own …

…file (carbon-language#4922)

context.cpp is getting large, so I'm looking at a few ways to cut out
clusters of functions. This felt like a logical cluster of functions to
move to their own file.

Note I have two commits in this PR: one moving the functionality to a
new file, and one specifically changing TypeCompleter to use out-of-line
function implementations. This is to assist reviewability.

toolchain/check/BUILD

@@ -39,6 +39,7 @@ cc_library(
         "pattern_match.cpp",
         "return.cpp",
         "subst.cpp",
+        "type_completion.cpp",
     ],
     hdrs = [
         "call.h",
@@ -70,6 +71,7 @@ cc_library(
         "pending_block.h",
         "return.h",
         "subst.h",
+        "type_completion.h",
     ],
     deps = [
         ":node_stack",