Rename semantic to sem

* Rename namespace semantic to sem
* Rename directory src/semantic/ to src/sem/

Bug: tint:724
Change-Id: I76383b821fbca7f1037a803c497b595a706dcb06
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48120
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/docs/arch.md b/docs/arch.md
index de7fca7..d4af10b 100644
--- a/docs/arch.md
+++ b/docs/arch.md
@@ -115,7 +115,7 @@
 
 ## Semantic information
 
-Semantic information is held by `semantic::Node`s which describe the program at
+Semantic information is held by `sem::Node`s which describe the program at
 a higher / more abstract level than the AST. This includes information such as
 the resolved type of each expression, the resolved overload of an intrinsic
 function call, and the module scoped variables used by each function.
@@ -123,11 +123,11 @@
 Semantic information is generated by the `Resolver` when the `Program`
 is built from a `ProgramBuilder`.
 
-The `semantic::Info` class holds a map of `ast::Node`s to `semantic::Node`s.
+The `sem::Info` class holds a map of `ast::Node`s to `sem::Node`s.
 This map is **many-to-one** - i.e. while a AST node might have a single
 corresponding semantic node, the reverse may not be true. For example:
-many `ast::IdentifierExpression` nodes may map to a single `semantic::Variable`,
-and so the `semantic::Variable` does not have a single corresponding
+many `ast::IdentifierExpression` nodes may map to a single `sem::Variable`,
+and so the `sem::Variable` does not have a single corresponding
 `ast::Node`.
 
 Unlike `ast::Node`s, semantic nodes may not necessarily form a directed acyclic