add --enable-debuginfo-tests, analogous to --disable-optimize-tests.

Then, decouple the question of whether the compiler/stdlib carry
debuginfo (which is controlled via `--enable-debuginfo` and implied by
`--enable-debug`) from the question of whether the tests carry
debuginfo (which now no longer is implied by `--enable-debug` nor
`--enable-debuginfo`, and is off by default).

Author: pnkfelix

Diff for: configure

@@ -551,6 +551,7 @@ opt valgrind-rpass 1 "run rpass-valgrind tests with valgrind"
 opt docs     1 "build standard library documentation"
 opt compiler-docs     0 "build compiler documentation"
 opt optimize-tests 1 "build tests with optimizations"
+opt debuginfo-tests 0 "build tests with debugger metadata"
 opt libcpp 1 "build with llvm with libc++ instead of libstdc++ when using clang"
 opt llvm-assertions 0 "build LLVM with assertions"
 opt debug-assertions 0 "build with debugging assertions"

Diff for: mk/tests.mk

@@ -632,6 +632,13 @@ ifndef CFG_DISABLE_OPTIMIZE_TESTS
 CTEST_RUSTC_FLAGS += -O
 endif
 
+# Analogously to the above, whether to pass `-g` when compiling tests
+# is a separate choice from whether to pass `-g` when building the
+# compiler and standard library themselves.
+CTEST_RUSTC_FLAGS := $$(subst -g,,$$(CTEST_RUSTC_FLAGS))
+ifdef CFG_ENABLE_DEBUGINFO_TESTS
+CTEST_RUSTC_FLAGS += -g
+endif
 
 CTEST_COMMON_ARGS$(1)-T-$(2)-H-$(3) := \
 		--compile-lib-path $$(HLIB$(1)_H_$(3)) \