MethodBodyStreamEncoder.AddMethodBody ignores attributes for tiny methods.

[VSadov]

RelatedTo:dotnet/roslyn#23951

MethodBodyStreamEncoder.AddMethodBody checks if it is possible to emit Tiny method header, and if it otherwise fits in Tiny, InitLocals flag get effectively ignored.

There is a case where we may need Fat format specifically because we want the InitLocals flag.
That happens if a method otherwise meets requirements of a Tiny format, but has stackalloc in it. If Tiny is used in such case, the caller will stackalloc a piece of nondeterministic garbage.

Overall - it should be up to the caller to decide whether the InitLocals is needed.
AddMethodBody should ensure that the flag is emitted if asked by the caller.

[VSadov]

CC:@tmat

[VSadov]

CC:@agocke, @t-camaia

For the time being, if we need to force the presence of the initlocals bit, we can workaround this by specifying stack size that is too large for a Tiny header.

Anything greater than 8 will work.

[stephentoub]

@tmat, your PR dotnet/corefx#27589 closed this issue, but there are still two tests disabled against it:

runtime/src/libraries/System.Reflection.Metadata/tests/Metadata/Ecma335/Encoding/MethodBodyStreamEncoderTests.cs

Lines 377 to 378 in fd7a5e8

	[Fact, ActiveIssue("https://github.com/dotnet/corefx/issues/26910")]
	public unsafe void LocAlloc()

runtime/src/libraries/System.Reflection.Metadata/tests/Metadata/Ecma335/Encoding/MethodBodyStreamEncoderTests.cs

Lines 422 to 423 in fd7a5e8

	[Fact, ActiveIssue("https://github.com/dotnet/corefx/issues/26910")]
	public unsafe void LocAlloc_WithInstructionEncoder()

[ericstj]

Moving to 6.0.0

Remaining work is to re-enable these tests.