[compiler-rt] Avoid memintrinsic calls inserted by the compiler

[compiler-rt] Avoid memintrinsic calls inserted by the compiler

D135716 introduced -ftrivial-auto-var-init=pattern where supported.
Unfortunately this introduces unwanted memset() for large stack arrays,
as shown by the new tests added for asan and msan (tsan already had this
test).

In general, the problem of compiler-inserted memintrinsic calls
(memset/memcpy/memmove) is not new to compiler-rt, and has been a
problem before.

To avoid introducing unwanted memintrinsic calls, we redefine
memintrinsics as __sanitizer_internal_mem* at the assembly level for
most source files automatically (where sanitizer_common_internal_defs.h
is included).

In few cases, redefining a symbol in this way causes issues for
interceptors, namely the memintrinsic interceptor themselves. For such
source files we have to selectively disable the redefinition.

Other alternatives have been considered, but simply do not work well in
the context of compiler-rt:

	1. Linker --wrap:  this does not work because --wrap only
	   applies to the final link, and would not apply when building
	   sanitizer static libraries.

	2. Changing references to memset() via objcopy:  this may work,
	   but due to the complexities of the build system, introducing
	   such a post-processing step for the right object files (in
	   particular object files defining memset cannot be touched)
	   seems infeasible.

The chosen solution works well (as shown by the tests). Other libraries
have chosen the same solution where nothing else works (see e.g. glibc's
"symbol-hacks.h").

v2:
- Fix ubsan_minimal build where compiler decides to insert
  memset/memcpy: ubsan_minimal has work without RTSanitizerCommonLibc,
  therefore do not redefine the builtins.
- Fix definition of internal_mem* functions with compilers that want the
  aliased function to already be defined before.
- Fix definition of __sanitizer_internal_mem* functions with compilers
  more pedantic about attribute placement around extern "C".

Reviewed By: vitalybuka, dvyukov

Differential Revision: https://reviews.llvm.org/D151152