Merge #1137

1137: ast: lower: Split rust-lower-base in its own source file r=CohenArthur a=CohenArthur

The implementation of all functions declared in `rust-ast-lower-base.h` was a big offender of our "definitions in headers" problem, spanning over 900 lines of code. We can do the same for basically all headers in `gcc/rust/hir` which all contain some definitions that have accumulated over time

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>

Author: bors[bot]

gcc/rust/Make-lang.in

@@ -83,6 +83,7 @@ GRS_OBJS = \
     rust/rust-attributes.o \
     rust/rust-abi.o \
     rust/rust-ast-lower.o \
+    rust/rust-ast-lower-base.o \
     rust/rust-ast-lower-pattern.o \
     rust/rust-ast-resolve.o \
     rust/rust-ast-resolve-pattern.o \